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

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)

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

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

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

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

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

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

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

Pressure solution (diffusion flow) in sandstone…

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

Diffusion Creep Strongly grain size sensitive Grain size not reduced

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

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

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

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

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 ~700-800 °C Veins/cracks at 500-550 °C, W. Gneiss, Norway