DeformationDeformation Mechanisms:Mechanisms: WhatWhat strainstrain occurredoccurred inin thisthis rock?rock? OutlineOutline

Main Mechanisms and Factors:

1. Microfracturing, Cataclasis, and Frictional Sliding

2. Mechanical Twinning and Kinking

3. Diffusion Creep

4. Dissolution Creep

5. Dislocation Creep MainMain MechanismsMechanisms andand FactorsFactors

Processes that permit rocks to deform at microscopic and atomic scales:

• Differential Stress and Temperature PotentialPotential FactorsFactors

• Mineralogy

• Grain size

• Temperature

• Differential stress

• Confining pressure

• Strain rate

• Fluid (or lack of); fluid pressure

• Constructive and destructive effects MicrofracturingMicrofracturing,, CataclasisCataclasis && FrictionalFrictional SlidingSliding

• 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 MechanicalMechanical TwinningTwinning && KinkingKinking

• Bending of the crystalline lattice without brittle failure

• Lattice is deformed along discrete planes CreepCreep

• A slow, time-dependent strain

• Differential stresses are not great enough to produce brittle failure

• The Three Creeps - Diffusion, Dissolution, Dislocation DiffusionDiffusion CreepCreep

• 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 ThreeThree TypesTypes ofof DiffusionDiffusion CreepCreep

•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 DissolutionDissolution CreepCreep DissolutionDissolution CreepCreep DissolutionDissolution CreepCreep DislocationDislocation CreepCreep

• Distortion of the crystal lattice on a slip planes

• Bonds progressively break along the slip plane DislocationDislocation CreepCreep DislocationDislocation CreepCreep DislocationDislocation CreepCreep DislocationDislocation CreepCreep DislocationDislocation CreepCreep DislocationDislocation CreepCreep RecoveryRecovery andand RecrystallizationRecrystallization

• 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 RecoveryRecovery

• Dislocation climb - rearrangement of dislocations RecrystallizationRecrystallization ExampleExample

100 µm

Undeformed Black Hills Quartzite (average grain size 100 µm) RecrystallizationRecrystallization

100 µm

50% shortening, 800°C, 1200 MPa, ~0.2% wt. H2O Dislocation creep is occurring RecrystallizationRecrystallization

100 µm

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

100 µm 60% shortening, 800°C, 1200 MPa, 120 hrs at 900°C

Recrystallization and annealing complete ReferencesReferences

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 22 http://talc.geo.umn.edu/orgs/struct/microstructure/images/005.html

Slide 23 http://talc.geo.umn.edu/orgs/struct/microstructure/images/006.html

Slide 24 http://talc.geo.umn.edu/orgs/struct/microstructure/images/010.html

Slide 25 http://talc.geo.umn.edu/orgs/struct/microstructure/images/014.html