Deformation Mechanisms: What strain Deformation Mechanisms: What strain occurred in this rock?occurred in this rock?

OutlineOutline

Main Mechanisms and Factors:Main Mechanisms and Factors:

1.1. Microfracturing, Cataclasis, and Frictional SlidingMicrofracturing, Cataclasis, and Frictional Sliding

2.2. Mechanical Twinning and KinkingMechanical Twinning and Kinking

3.3. Diffusion CreepDiffusion Creep

4.4. Dissolution CreepDissolution Creep

5.5. Dislocation CreepDislocation Creep

Main Mechanisms and Main Mechanisms and

FactorsFactors

• Differential Stress and TemperatureDifferential Stress and Temperature

Processes that permit Processes that permit

rocks to deform at rocks to deform at

microscopic and atomic microscopic and atomic

scales:scales:

Potential FactorsPotential Factors

• MineralogyMineralogy

• Grain sizeGrain size

• TemperatureTemperature

• Differential stressDifferential stress

• Confining pressureConfining pressure

• Strain rateStrain rate

• Fluid (or lack of); fluid pressureFluid (or lack of); fluid pressure

• Constructive and destructive effectsConstructive and destructive effects

Microfracturing, Cataclasis & Frictional Microfracturing, Cataclasis & Frictional

SlidingSliding• Brittle deformation on the grain Brittle deformation on the grain to subgrain scaleto subgrain scale

• Development, propagation and Development, propagation and slip of microcracksslip of microcracks

• Frictional sliding and flow of Frictional sliding and flow of crushed rock & crystal material crushed rock & crystal material ((Cataclastic FlowCataclastic Flow) along grain ) along grain boundariesboundaries

Mechanical Twinning & KinkingMechanical Twinning & Kinking

• Bending of the crystalline Bending of the crystalline lattice without brittle failurelattice without brittle failure

• Lattice is deformed along Lattice is deformed along discrete planesdiscrete planes

CreepCreep

• A slow, time-dependent strainA slow, time-dependent strain

• Differential stresses are not great enough to produce brittle Differential stresses are not great enough to produce brittle failurefailure

• The The ThreeThree Creeps - Diffusion, Dissolution, Dislocation Creeps - Diffusion, Dissolution, Dislocation

Diffusion CreepDiffusion Creep

• Influenced by average kinetic energy (temperature)Influenced by average kinetic energy (temperature)

• A vacancy or defect needs to occur for atoms to move A vacancy or defect needs to occur for atoms to move through the crystal latticethrough the crystal lattice

• Atoms can move through grains, along grain boundaries, Atoms can move through grains, along grain boundaries, and through pore space (with fluid present)and through pore space (with fluid present)

• The presence of fluids speed up diffusion creep The presence of fluids speed up diffusion creep

Three Types of Diffusion Three Types of Diffusion

CreepCreep•Volume-diffusion creepVolume-diffusion creep - diffusion occurring within a grain - diffusion occurring within a grain

•Grain-boundary diffusion creepGrain-boundary diffusion creep - diffusion occurring along a - diffusion occurring along a

grain boundarygrain boundary

•Superplastic creepSuperplastic creep - grain-boundary sliding and grain-boundary - grain-boundary sliding and grain-boundary

diffusiondiffusion

Dissolution CreepDissolution Creep

Dissolution CreepDissolution Creep

Dissolution CreepDissolution Creep

Dislocation CreepDislocation Creep

• Distortion of the crystal lattice on a slip planesDistortion of the crystal lattice on a slip planes

• Bonds progressively break along the slip planeBonds progressively break along the slip plane

Dislocation CreepDislocation Creep

Dislocation CreepDislocation Creep

Dislocation CreepDislocation Creep

Dislocation CreepDislocation Creep

Dislocation CreepDislocation Creep

Dislocation CreepDislocation Creep

Recovery and Recovery and

RecrystallizationRecrystallization• To “repair” dislocations, the crystal structure must be returned To “repair” dislocations, the crystal structure must be returned to the previous state ( i.e., no dislocations)to the previous state ( i.e., no dislocations)

• RecoveryRecovery - rearrangement and destruction of dislocations - rearrangement and destruction of dislocations

• Recrystallization and neomineralizationRecrystallization and neomineralization - transformation of - transformation of old “defective” grains into brand-new grains or new old “defective” grains into brand-new grains or new configurations of grains:configurations of grains:

• Rotation of grain boundariesRotation of grain boundaries

• Migration of grain boundariesMigration of grain boundaries

• Dynamic recrystallizationDynamic recrystallization - recovery and recrystallization - recovery and recrystallization during deformationduring deformation

• AnnealingAnnealing - recovery and recrystallization after deformation - recovery and recrystallization after deformation

RecoveryRecovery

• Dislocation climb -

rearrangement of

dislocations

Recrystallization ExampleRecrystallization Example

Undeformed Black Hills Quartzite (average grain size 100 Undeformed Black Hills Quartzite (average grain size 100 m)m)

100 100 mm

RecrystallizationRecrystallization

100 100 mm

50% shortening, 800°C, 1200 MPa, ~0.2% wt. H50% shortening, 800°C, 1200 MPa, ~0.2% wt. H22OO

Dislocation creep is occurringDislocation creep is occurring

RecrystallizationRecrystallization

100 100 mm

57% shortening, 900°C, 1200 MPa, ~0.2% wt. H57% shortening, 900°C, 1200 MPa, ~0.2% wt. H22OO

Recrystallization is occurringRecrystallization is occurring

RecrystallizationRecrystallization

100 100 mm

60% shortening, 800°C, 1200 MPa, 120 hrs at 900°C60% shortening, 800°C, 1200 MPa, 120 hrs at 900°C

Recrystallization and annealing completeRecrystallization and annealing complete

ReferencesReferences

Slide 1Slide 1http://talc.geo.umn.edu/orgs/struct/microstructure/images/024.html

Slides 3, 5 - 19, 21Slides 3, 5 - 19, 21Davis. G. H. and S. J. Reynolds, Structural Geology of Rocks and Regions, 2nd ed., John Davis. G. H. and S. J. Reynolds, Structural Geology of Rocks and Regions, 2nd ed., John Wiley & Sons, New York, 776 p., 1996.Wiley & Sons, New York, 776 p., 1996.

Slide 13Slide 13Scholz, C. H., The Mechanics of Earthquakes and Faulting, 2nd. ed., Cambridge Scholz, C. H., The Mechanics of Earthquakes and Faulting, 2nd. ed., Cambridge University Press, 471 p., 2002.University Press, 471 p., 2002.

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

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

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

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