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. cataclasite
Alpine Fault

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 Effective 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
( )
“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 pre-exponential energy
factor
activation
energy
stress
exponent
strain rate
differential
stress
universal
gas constant
= 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
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