Presentation on theme: "Ductile deformational processes"— Presentation transcript:
1Ductile deformational processes Introduction: how can rocks bend, distort, or flow while remaining a solid?Non-recoverable deformation versus elastic deformationThree mechanisms:1) Catalclastic flow2) Diffusional mass transfer3) Crystal plasticityControlled bytemperaturestressstrain rategrain size compositionfluid contentFYI, what is considered high-temp behavior for one mineral is low-temp. behavior for another mineral.Normalized paramater, homologous temperature, ThTh = T/Tm
2Ductile deformational processes Catalclastic flowCataclastic flow: rock fractured into smaller particles that slide/flow past one anotherLarge grain microfracture at grain boundary scale or within individual grainsShallow-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, ThTh = T/TmBeanbag experiment
3Ductile deformational processes Crystal defectsMotion of defects gives rise to permanent strain with the material losing cohesion (no fracturing)Ductile behavior at elevated temperaturesAchieved by motion of crystal defects (error in crystal lattice)Point defectsLine defects or dislocationsPlanar defects
4Ductile deformational processes Crystal defectsVacancies – unoccupied siter in the crystal lattice and replaced by different atom (b)(c) Or by interstitails, atom at a non-lattice siteVacancies can migrate by exchange with atoms at neighboring sites (d) – also called diffusionPoint defectsTwo types:Vacancies & Impurities
5Ductile deformational processes Crystal defects2) Line defectsAlso called a dislocation – a linear array of lattice imperfections.Two end-member configurations.Difficult conceptAn area of the crystal that has slipped relative to the rest of the crystal.
6Ductile deformational processes Crystal defectsTwo end-member configurations.Edge dislocation: extra half-plane of atoms in the latticeAn area of the crystal that has slipped relative to the rest of the crystal.
7Ductile deformational processes Crystal defectsTwo end-member configurations.A) Screw dislocation: atoms are deformed in a crew-like fashionAn area of the crystal that has slipped relative to the rest of the crystal.
8Deformation Mechanisms Important relationsNormalized stress (normalized to shear modulus of the materialversusnormalized temperature (normalized to absolute melting temperature of the material)An area of the crystal that has slipped relative to the rest of the crystal.
9Deformation Mechanisms Important relationsDifferential stressversusTemperatureAn area of the crystal that has slipped relative to the rest of the crystal.
10Deformation 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 sizeTemperatureConfining and fluid pressureDifferential stress (s1 - s3)Strain rateIn most polymineralic rocks, a number of different defm. mechanisms will be at work simultaneously.If conditions change during the deformation so will the mechanisms.
11The 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.
12Deformation Mechanism Map CataclasisDissolution creepDislocation creepDiffusion creepPressure solutionEach of thesemechanisms can bedominant in the creep ofrocks, depending on thetemperature anddifferential stressconditions.Depth / Temperature
13Fine-scale fracturing, movement along fractures and frictional grain-boundary sliding. Favoured by low-confining pressuresCauses decrease in porosity and rock volume.
14Microfracturing, 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.
16Microcracks break individual atomic bonds Crack tips have nearly infinitesimally small areas, which makes the stresses there HUGE!
17Mechanical 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 and2) The twin plane must be favorable oriented.
19Kinking 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.
20Diffusion: atom jump from site to site through a mineral. DissolutionDislocationDiffusion: 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.
21Volume-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.