Ductile deformational processes de

1. 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 stress strain rate grain size composition fluid content

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) Beanbag experiment

3. Ductile deformational processes Crystal defects • 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 • 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

6. Ductile deformational processes Crystal defects • Two end-member configurations. • Edge dislocation: extra half-plane of atoms in the lattice

7. Ductile deformational processes Crystal defects Two end-member configurations. A) Screw dislocation: atoms are deformed in a crew-like fashion

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)

9. Deformation Mechanisms Important relations Differential stress versus Temperature

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.

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.

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 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.

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.