Transcript lecture9_ductile_def

Ductile deformational processes
Introduction: how can rocks bend, distort, or flow while remaining a solid?
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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
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)
Beanbag experiment
Ductile deformational processes
Crystal defects
Ductile behavior at elevated temperatures
Achieved by motion of crystal defects (error in crystal lattice)
1)Point defects
2)Line defects or dislocations
3)Planar defects
Ductile deformational processes
Crystal defects
1) Point defects
Two types:
Vacancies & Impurities
Ductile deformational processes
Crystal defects
2) Line defects
Also called a dislocation – a linear
array of lattice imperfections.
Two end-member configurations.
Difficult concept
Ductile deformational processes
Crystal 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: atoms are deformed in a crew-like fashion
Deformation Mechanisms
Important relations
Normalized stress (normalized to
shear modulus of the material
versus
normalized temperature
(normalized to absolute melting
temperature of the material)
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 (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.
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
• 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.