Ductile deformational processes

deIntroduction: how can rocks bend, distort, or flow while remaining a solid?

Non-recoverable deformation versus elastic deformation

Three mechanisms:1) Catalclastic flow2) Diffusional mass transfer3) Crystal plasticity

Controlled by temperaturestressstrain rategrain size compositionfluid content

Ductile deformational processes

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)

Catalclastic flow

Beanbag experiment

Ductile deformational processes

Ductile behavior at elevated temperaturesAchieved by motion of crystal defects (error in crystal lattice)

1)Point defects2)Line defects or dislocations3)Planar defects

Crystal defects

Ductile deformational processes

1) Point defects

Two types: Vacancies & Impurities

Crystal defects

Ductile deformational processes

2) Line defects

Also called a dislocation – a linear array of lattice imperfections.

Two end-member configurations.

Difficult concept

Crystal defects

Ductile deformational processesCrystal defects

Two end-member configurations.

A) Edge dislocation: extra half-plane of atoms in the lattice

Ductile deformational processesCrystal defects

Two end-member configurations.

A) Screw dislocation: atoms are deformed in a crew-like fashion

Deformation MechanismsImportant relations

Normalized stress (normalized to shear modulus of the material

versus

normalized temperature (normalized to absolute melting temperature of the material)

Deformation MechanismsImportant 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.

Deformation Mechanism Map

Dep

th /

Te

mpe

ratu

re

CataclasisDissolution creepDislocation creepDiffusion creepPressure solution

Each of thesemechanisms can bedominant in the creep ofrocks, depending on thetemperature anddifferential stressconditions.

• Fine-scale fracturing, movement along fractures and frictional grain-boundary sliding.

• Favoured by low-confining pressures

• Causes decrease in porosity and rock volume.

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.

Microcrack in Feldspar

Microcracks break individual atomic bonds

Crack tips have nearly infinitesimally small areas, which makes the stresses there HUGE!

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.

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.

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.

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.