Deformation Mechanisms: What strain occurred in this rock?

Outline Main Mechanisms and Factors: Main Mechanisms and Factors: 1. Microfracturing, Cataclasis, and Frictional Sliding 2. Mechanical Twinning and Kinking 3. Diffusion Creep 4. Dissolution Creep 5. Dislocation Creep

Main Mechanisms and Factors Differential Stress and Temperature Differential Stress and Temperature Processes that permit rocks to deform at microscopic and atomic scales:

Potential Factors Mineralogy Mineralogy Grain size Grain size Temperature Temperature Differential stress Differential stress Confining pressure Confining pressure Strain rate Strain rate Fluid (or lack of); fluid pressure Fluid (or lack of); fluid pressure Constructive and destructive effects Constructive and destructive effects

Microfracturing, Cataclasis & Frictional Sliding Brittle deformation on the grain to subgrain scale Brittle deformation on the grain to subgrain scale Development, propagation and slip of microcracks Development, propagation and slip of microcracks Frictional sliding and flow of crushed rock & crystal material (Cataclastic Flow) along grain boundaries Frictional sliding and flow of crushed rock & crystal material (Cataclastic Flow) along grain boundaries

Mechanical Twinning & Kinking Bending of the crystalline lattice without brittle failure Bending of the crystalline lattice without brittle failure Lattice is deformed along discrete planes Lattice is deformed along discrete planes

Creep A slow, time-dependent strain A slow, time-dependent strain Differential stresses are not great enough to produce brittle failure Differential stresses are not great enough to produce brittle failure The Three Creeps - Diffusion, Dissolution, Dislocation The Three Creeps - Diffusion, Dissolution, Dislocation

Diffusion Creep Influenced by average kinetic energy (temperature) Influenced by average kinetic energy (temperature) A vacancy or defect needs to occur for atoms to move through the crystal lattice 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) Atoms can move through grains, along grain boundaries, 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 Creep Volume-diffusion creep - diffusion occurring within a grain Volume-diffusion creep - diffusion occurring within a grain Grain-boundary diffusion creep - diffusion occurring along a grain boundary Grain-boundary diffusion creep - diffusion occurring along a grain boundary Superplastic creep - grain-boundary sliding and grain- boundary diffusion Superplastic creep - grain-boundary sliding and grain- boundary diffusion

Dissolution Creep

Dislocation Creep Distortion of the crystal lattice on a slip planes Distortion of the crystal lattice on a slip planes Bonds progressively break along the slip plane Bonds progressively break along the slip plane

Dislocation Creep

Recovery and Recrystallization To “repair” dislocations, the crystal structure must be returned to the previous state ( i.e., no dislocations) To “repair” dislocations, the crystal structure must be returned to the previous state ( i.e., no dislocations) Recovery - rearrangement and destruction of dislocations Recovery - rearrangement and destruction of dislocations Recrystallization and neomineralization - transformation of old “defective” grains into brand-new grains or new configurations of grains: Recrystallization and neomineralization - transformation of old “defective” grains into brand-new grains or new configurations of grains: Rotation of grain boundaries Rotation of grain boundaries Migration of grain boundaries Migration of grain boundaries Dynamic recrystallization - recovery and recrystallization during deformation Dynamic recrystallization - recovery and recrystallization during deformation Annealing - recovery and recrystallization after deformation Annealing - recovery and recrystallization after deformation

Recovery Dislocation climb - rearrangement of dislocations

Recrystallization Example Undeformed Black Hills Quartzite (average grain size 100  m) 100  m

Recrystallization 50% shortening, 800°C, 1200 MPa, ~0.2% wt. H 2 O Dislocation creep is occurring

Recrystallization 100  m 57% shortening, 900°C, 1200 MPa, ~0.2% wt. H 2 O Recrystallization is occurring

Recrystallization 100  m 60% shortening, 800°C, 1200 MPa, 120 hrs at 900°C Recrystallization and annealing complete

References Slide 1 Slides 3, , 21 Davis. G. H. and S. J. Reynolds, Structural Geology of Rocks and Regions, 2nd ed., John Wiley & Sons, New York, 776 p., Slide 13 Scholz, C. H., The Mechanics of Earthquakes and Faulting, 2nd. ed., Cambridge University Press, 471 p., Slide 22 Slide 22 Slide 23 Slide 23 Slide 24 Slide 24 Slide 25 Slide 25