1. High T Deformation Mechanisms involved in Localization
Ge277, 2011, Steve Kidder

2. Outline Dislocation Creep Diffusion Creep Grain Boundary Sliding Other
Caviat: These mechanisms and constituitive relationships have generally been developed for monomineralic aggregates, while most crust contains multiple minerals
Salinas shear zone, Kidder & Ducea (2006)

3. 1. Dislocation Creep involves dislocations...
(Translation of a stressed edge dislocation)

4. TEM image of tangled dislocations in quartz
(2x diameter of the Earth)
(FOV ~ 1 µm?)
Typical density 2.6x109 cm/cm^3

5. Dislocation Creep
Grain size evolves to a constant (generally smaller) size proportional to differential stress
Viscosity not grain size sensitive
Lattice Preferred Orientation can reduce viscosity (e.g. by 10x in qtz; Muto, 2011)
Stress exponent varies
A includes water fugacity term

6. Undeformed Black Hills Sandstone (BHQ), cross polars
Cross polarized light. Color reflects grain orientation
~50 microns

7. BHQ (fast strain rate, 10^-4 /s), note strong lattice distortion, tiny rxl grains
~50 microns

8. Recrystallized Grain Size proportional to Differential stress
Empirical relationship observed in materials deformed in dislocation creep regime:
σd = kdm
d = recrystallized grain diameter
m (= ~-1) and k experimentally determined
Stipp and Tullis, 2003

9. 2. Diffusion Creep (incl. pressure soln.)
Deformation by migration of point defects, most commonly along grain boundaries

10. Pressure solution (diffusion flow) in sandstone…

11. Pressure Solution: cleavage (~ horizontal) cutting lithological layering
TN 128a, scale bar uncertain
~1 mm

12. Diffusion Creep
Strongly grain size sensitive
Grain size not reduced

13. 3. Grain Boundary Sliding assisted Disl. Creep
Precigout et al. 2007

14. 3. Grain Boundary Sliding assisted Dislocation Creep
Both reduces grain size, and grain size weakening
Documented (so far) only in Olivine & Ice

15. Comparison Disl. creep: Diff. creep: GBS Disl. creep:
Without GBS there isn’t generally a way to weaken rocks substantially using only Disl and Diff creep

16. GBS assisted Disl. Creep helps explain localization
1D numerical experiments indicate a 2 order of mag. increase in strain rate associated with localization
Precigout & Gueydan. 2009
Classical view

17. Other microscale mechanisms involved in typical shear crustal zones
Microcracking
Metamorphic changes
Grain boundary sliding
Partial melting
(all can assist in localization)
Salinas shear zone, Kidder & Ducea (2006)
Veins/cracks at ~ °C
Veins/cracks at °C, W. Gneiss, Norway