Deformation Mechanisms: What strain occurred in this rock?

1. Deformation Mechanisms: What strain occurred in this rock?

2. Outline Main Mechanisms and Factors: • Microfracturing, Cataclasis, and Frictional Sliding • Mechanical Twinning and Kinking • Diffusion Creep • Dissolution Creep • Dislocation Creep

3. Main Mechanisms and Factors Processes that permit rocks to deform at microscopic and atomic scales: • Differential Stress and Temperature

4. Potential Factors • Mineralogy • Grain size • Temperature • Differential stress • Confining pressure • Strain rate • Fluid (or lack of); fluid pressure • Constructive and destructive effects

5. Microfracturing, Cataclasis & Frictional Sliding • Brittle deformation on the grain to subgrain scale • Development, propagation and slip of microcracks • Frictional sliding and flow of crushed rock & crystal material (Cataclastic Flow) along grain boundaries

6. Mechanical Twinning & Kinking • Bending of the crystalline lattice without brittle failure • Lattice is deformed along discrete planes

7. Creep • A slow, time-dependent strain • Differential stresses are not great enough to produce brittle failure • The Three Creeps - Diffusion, Dissolution, Dislocation

8. Diffusion Creep • Influenced by average kinetic energy (temperature) • A vacancy or defect needs to occur for atoms to move through the crystal lattice • Atoms can move through grains, along grain boundaries, and through pore space (with fluid present) • The presence of fluids speed up diffusion creep

9. Three Types of Diffusion Creep • Volume-diffusion creep - diffusion occurring within a grain • Grain-boundary diffusion creep- diffusion occurring along a grain boundary • Superplastic creep - grain-boundary sliding and grain-boundary diffusion

10. Dissolution Creep

11. Dissolution Creep

12. Dissolution Creep

13. Dislocation Creep • Distortion of the crystal lattice on a slip planes • Bonds progressively break along the slip plane

14. Dislocation Creep

15. Dislocation Creep

16. Dislocation Creep

17. Dislocation Creep

18. Dislocation Creep

19. Dislocation Creep

20. Recovery and Recrystallization • To “repair” dislocations, the crystal structure must be returned to the previous state ( i.e., no dislocations) • Recovery - rearrangement and destruction of dislocations • Recrystallization and neomineralization - transformation of old “defective” grains into brand-new grains or new configurations of grains: • Rotation of grain boundaries • Migration of grain boundaries • Dynamic recrystallization - recovery and recrystallization during deformation • Annealing - recovery and recrystallization after deformation

21. Recovery • Dislocation climb - rearrangement of dislocations

22. Recrystallization Example 100 mm Undeformed Black Hills Quartzite (average grain size 100 mm)

23. Recrystallization 100 mm 50% shortening, 800°C, 1200 MPa, ~0.2% wt. H2O Dislocation creep is occurring

24. Recrystallization 100 mm 57% shortening, 900°C, 1200 MPa, ~0.2% wt. H2O Recrystallization is occurring

25. Recrystallization 100 mm 60% shortening, 800°C, 1200 MPa, 120 hrs at 900°C Recrystallization and annealing complete

26. References Slide 1 http://talc.geo.umn.edu/orgs/struct/microstructure/images/024.html Slides 3, 5 - 19, 21 Davis. G. H. and S. J. Reynolds, Structural Geology of Rocks and Regions, 2nd ed., John Wiley & Sons, New York, 776 p., 1996. Slide 13 Scholz, C. H., The Mechanics of Earthquakes and Faulting, 2nd. ed., Cambridge University Press, 471 p., 2002. Slide 22http://talc.geo.umn.edu/orgs/struct/microstructure/images/005.html Slide 23http://talc.geo.umn.edu/orgs/struct/microstructure/images/006.html Slide 24http://talc.geo.umn.edu/orgs/struct/microstructure/images/010.html Slide 25http://talc.geo.umn.edu/orgs/struct/microstructure/images/014.html