Microscopic aspects of rock deformation (Part I)

1. Microscopic aspects of rock deformation (Part I) ESCI 302

3. Deformation mechanisms • Operative mechanisms depend on: • Temperature (T) • Lithostatic pressure (plitho) • Fluid pressure (pf) • Differential stress (σd = σ1- σ3) • Strain rate ( ) • Porosity (Φ) • Grain size (d) • Chemical conditions • Rock type

4. Deformation mechanisms • Tell us about bulk rheology (e.g., power-law creep) • Result in the grain-scale fabric & microstructures of metamorphic rocks (e.g., crystallographic preffered orientations) • Tell us about physical conditions during deformation (e.g. T, plitho, pf etc.)

5. Deformation mechanisms • Upper crust: • Brittle deformation dominant • (also elastic deformation) • Mid to lower crust: • Elastic deformation, ± brittle failure (minor), • Dislocation creep (“crystal plasticity”) • Other diffusion controlled “ductile” mechanisms

6. Two classes of def. mechs. • Elastic-recoverable (ε < 2-3%) • Recoverable • Not permanent • Instantaneous • No breaking of bonds • Non-recoverable (ε > 2-3%) • Permanent • Time-dependant, non-instantaneous (rock strength related to ) • Bonds broken

7. DM: early to pre-diagenesis • Intergranular flow (particle flow) • rolling and sliding of rigid particles past each other • in unlithified sediments • low confining pressures and/or high fluid pressures • no microstructures

9. DM: fault zones near brittle-ductile transition • Cataclastic flow: • high effective pressures (e.g., depths >6 km) • involves continuous brittle fracturing of grains in a rock • produces progressive decrease in grain size • different from granular flow in that small-scale fracturing plays an integral role • little/no imprint on microfabric (no actual deformation of mineral grains except fracturing)

10. Alpine Fault cataclasite

12. Example: compaction • due to sediment burial (increases load) • involves partial removal of fluid phase from porous solid +/- removal of void space in response to sediment load • process yields porosity decrease with burial depth (non-linear) • May involve significant inter-granular flow and/or cataclastic flow • Little microfabric evidence (except fissilty and pencil structures in shale)

13. Example: compaction • Porosity: • fraction of void space in rocks void solid

14. Example: compaction • Fluid pressure • works against lithostatic pressure • reduces effective normal stress (impedes compaction) • Effective pressure plitho plitho pf plitho plitho

15. Compaction in wet rocks • Terzaghi’s spring analogue Karl von Terzaghi (1883-1963) “Father of soil mechanics” 1 2a 2b 3 underconsolidated consolidated time

16. Compaction in wet rocks • Terzaghi’s spring analogue fluid supports extra weight extra fluid pressure fluid at rest A 1 2a 2b 3 underconsolidated consolidated time

17. Compaction in wet rocks • Terzaghi’s spring analogue Weight transferred from fluid to load-bearing grains. Extra weight supported by compacted grains 1 2a 2b 3 underconsolidated consolidated time

18. Compaction in dry rocks • Terzaghi’s spring analogue 1 2

19. Compaction – reality check • actual rock behaviour is not spring-like, i.e. not elastic • Real compaction leads to permanent deformation (intergranular flow, fracturing etc) • time scale of compaction: 10s – 100s m.y. • pore closure by deformation and / or cementation of sediments yields “plugged system.” This causes retained high fluid pressures (Pf> Phydro) at depths of several km (i.e., pressurized fluid cannot escape).

20. Fluid pressure gradients • Lithostatic vs. hydrostatic Pf • Fluid pressure transition zone between two end-members • State of undercompaction / under-consolidation (problem in petroleum industry) Fluid Pressure ratio l = Pf/Plitho CD/Ch 8/depth curves/hydro-litho

21. Sources of fluids • Compaction-related pore fluids: connate brines • Water incorporated into a sediment during sub-aqueous deposition • Fluids released from minerals by dehydration during metamorphism • Adsorbed water (e.g. low grade smectite clays)

22. DM: diffusion and dislocation creep

23. Steady-state creep • Remember me? activation energy pre-exponential factor stress exponent strain rate differential stress universal gas constant = 8.314 J K-1 mol-1 temperature

24. Deformation mechanism maps • conditions of stress and temperature for some flow mechanisms can be plotted on a deformation mechanism map • deformation mechanism maps… … are lab-derived … are often monomineralic (e.g., “pure qtzite”) … are either “dry” or “wet” … are derived for 1 grain size

25. Deformation mechanism maps show fastest operative mechanism contours of strain-rate Flow-stress Flow-stress (normalized) Flow-stress (normalized) wet quartzite d = 100 µm calcite d = 100 µm Crystal plasticity Crystal plasticity Diffusion creep Diffusion creep Passchier & Trouw, 2005

26. Deformation mechanism maps • Problems: • only approximate, lab-derived • incorrect in detail (e.g. evidence from natural rocks suggests different def. mech. at certain stress or temperature level) • not all known mechanisms shown- only plot the fastest one despite others possibly operating together • not all conditions that natural rocks encounter are accounted for (e.g. hydrolytic weakening) • may not indicate effect of changing grain size • best to do: consider maps as a qualitative illustration of the relative importance of the different deformation mechanisms

27. DIY: deformation mechanism maps Passchier & Trouw 2005