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. MicrofracturingMicrofracturing,, CataclasisCataclasis, and Frictional Sliding, 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 FactorsMain Mechanisms and Factors

Processes that permit Processes that permit

rocks to deform at rocks to deform at

microscopic and atomic microscopic and atomic

scales:scales:

•• Differential Stress and TemperatureDifferential Stress and Temperature

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

MicrofracturingMicrofracturing, , Cataclasis Cataclasis & Frictional Sliding& Frictional Sliding

•• Brittle deformation on the grain Brittle deformation on the grain toto subgrain subgrain scalescale

•• Development, propagation and Development, propagation and slip of slip of microcracksmicrocracks

•• Frictional sliding and flow of Frictional sliding and flow of crushed rock & crystal material crushed rock & crystal material ((Cataclastic Cataclastic FlowFlow) 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, timeA slow, time--dependent straindependent strain

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

•• The The ThreeThree Creeps Creeps -- Diffusion, Dissolution, DislocationDiffusion, 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 CreepThree Types of Diffusion Creep

••VolumeVolume--diffusion creepdiffusion creep -- diffusion occurring within a graindiffusion occurring within a grain

••GrainGrain--boundary diffusion creepboundary diffusion creep -- diffusion occurring along a diffusion occurring along a

grain boundarygrain boundary

••Superplastic Superplastic creepcreep -- graingrain--boundary sliding and grainboundary sliding and grain--

boundary diffusionboundary diffusion

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 andRecovery and RecrystallizationRecrystallization

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

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

•• Recrystallization Recrystallization andand neomineralizationneomineralization -- transformation of transformation of old “defective” grains into brandold “defective” grains into brand--new grains or new new grains or new configurations of grains:configurations of grains:

•• Rotation of grain boundariesRotation of grain boundaries

•• Migration of grain boundariesMigration of grain boundaries

•• DynamicDynamic recrystallizationrecrystallization -- recovery andrecovery and recrystallization recrystallization during deformationduring deformation

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

RecoveryRecovery

• Dislocation climb -

rearrangement of

dislocations

RecrystallizationRecrystallization ExampleExample

100 100 µµmm

UndeformedUndeformed Black Hills Quartzite (average grain size 100 Black Hills Quartzite (average grain size 100 µµm)m)

RecrystallizationRecrystallization

100 100 µµmm

50% shortening, 800°C, 120050% shortening, 800°C, 1200 MPaMPa, ~0.2% wt. H, ~0.2% wt. H22OO

Dislocation creep is occurringDislocation creep is occurring

RecrystallizationRecrystallization

100 100 µµmm

57% shortening, 900°C, 120057% shortening, 900°C, 1200 MPaMPa, ~0.2% wt. H, ~0.2% wt. H22OO

RecrystallizationRecrystallization is occurringis occurring

RecrystallizationRecrystallization

100 100 µµmm

60% shortening, 800°C, 120060% shortening, 800°C, 1200 MPaMPa, 120 hrs at 900°C, 120 hrs at 900°C

RecrystallizationRecrystallization and annealing completeand annealing complete

ReferencesReferences

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

Slides 3, 5 Slides 3, 5 -- 19, 2119, 21Davis. G. H. and S. J. Reynolds, Structural Geology of Rocks andDavis. G. H. and S. J. Reynolds, Structural Geology of Rocks and Regions, 2nd ed., John Regions, 2nd ed., John Wiley & Sons, New York, 776 p., 1996.Wiley & Sons, New York, 776 p., 1996.

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

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

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

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

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