ductile deformational processes de
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Ductile deformational processes de. 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 - PowerPoint PPT PresentationTRANSCRIPT
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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
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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
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Ductile deformational processesCatalclastic flow
Franciscan, Rodeo cover thrust fault
Freenstone cataclasite
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Ductile deformational processesCatalclastic flow
Limestone cataclasite
Wasatch fault
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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
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Ductile deformational processes
1) Point defects
Two types: Vacancies & Impurities
Crystal defects
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Ductile deformational processes
2) Line defects
Also called a dislocation – a linear array of lattice imperfections.
Two end-member configurations.
Difficult concept
Crystal defects
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Ductile deformational processesCrystal defects
Two end-member configurations.
A) Edge dislocation: extra half-plane of atoms in the lattice
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Ductile deformational processesCrystal defects
Two end-member configurations.
A) Screw dislocation: atoms are deformed in a screw-like fashion
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Deformation MechanismsImportant relations
Normalized stress (normalized to shear modulus of the material
versus
normalized temperature (normalized to absolute melting temperature of the material)
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Deformation MechanismsImportant relations
Differential stress
versus
Temperature
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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.
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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.
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Deformation Mechanism Map
Dep
th /
Te
mpe
ratu
re
CataclasisDissolution creepDislocation creepDiffusion creepPressure solution
Each of thesemechanisms can bedominant in the creep of rocks, depending on the temperature and differential stressconditions.
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• Fine-scale fracturing, movement along fractures and frictional grain-boundary sliding.
• Favoured by low-confining pressures
• Causes decrease in porosity and rock volume.
Cataclasis
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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.
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Microcrack in Feldspar
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Microcracks break individual atomic bonds
Crack tips have nearly infinitesimally small areas, which makes the stresses there HUGE!
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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.
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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.
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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.
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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.