Lecture 16: Deformation mechanisms 1. Distribution of Deformation in the crust divide the crust into deformation regimes based on temperature and depth A Upper Crust(0-50 km) deformation localized along narrow zones or faults characterized by stick-slip seismic or creep behavior faults may exhibit both types of behavior(even on same fault) B Lower Crust(40-70 km) diffuse deformation spread over larger volume dominated by ductile flow processes strong influence of e, mineralogic changes, temperature and fluids C. Deformation Mechanism Maps sp show dominant modes of deformation under given stresses and temperatures 2. Low Temperature Deformation p occurs below homologous temperatures, Th=0.3Th=T/Tm) A. Brittle mechanisms and Particulate flow 1. frictional sliding: slip occurs when Coulomb criterion for sliding is met 2. granular flow: rolling and sliding of particles past and over one another 3. cataclastic flow: continuous brittle fracture and comminution of grains with some frictional sliding or rolling B. Twin Gliding p mechanical twinning of individual crystals along or across particular crystallographic lanes results in a kinked crystal geometry and can accommodate an overall shape change calcite twins at very low (-10 MPa)shear stress and low temperatures so this is an important deformation mechanisms in carbonate rocks C. Diffusive mass transfer processes deformation occurs by transfer of material from areas of high stress to areas of low stress 1. Pressure Solution(solution creep) dissolution of crystal surfaces and fluid-assisted transport of material to new sites of precipitation 2. Coble Creep diffusion of atoms from crystal surfaces in a dry environment deformation rates are linked to ease of diffusion and diffusion path lengths 3. High Temperature, Low to Moderate Stress Deformation ep dominated by diffusive mass transfer at higher temperatures we can now diffuse atoms or ions out from the interior of a crystal, instead of just the surface vacancies and interstitials in crystal lattice move in response to applied stress A Nabarro-Herring Creep vacancies move toward high stress surfaces while atoms move toward low stress surfaces 4. Deformation by Motion of Crystal Defects A. Crystal Defects 1. point defects 2. grain boundaries 16-1
Lecture 16: Deformation Mechanisms 16-1 1. Distribution of Deformation in the Crust divide the crust into deformation regimes based on temperature and depth A. Upper Crust (0 - 50 km) • deformation localized along narrow zones or faults - characterized by stick-slip seismic or creep behavior - faults may exhibit both types of behavior (even on same fault) B. Lower Crust (40 - 70 km) • diffuse deformation spread over larger volume - dominated by ductile flow processes - strong influence of sc , mineralogic changes, temperature and fluids C. Deformation Mechanism Maps show dominant modes of deformation under given stresses and temperatures 2. Low Temperature Deformation occurs below homologous temperatures, Th = 0.3 (Th = T/Tm ) A. Brittle Mechanisms and Particulate Flow 1. frictional sliding: slip occurs when Coulomb criterion for sliding is met 2. granular flow: rolling and sliding of particles past and over one another 3. cataclastic flow: continuous brittle fracture and comminution of grains with some frictional sliding or rolling B. Twin Gliding mechanical twinning of individual crystals along or across particular crystallographic planes results in a kinked crystal geometry and can accommodate an overall shape change of a rock • calcite twins at very low (~10 MPa) shear stress and low temperatures so this is an important deformation mechanisms in carbonate rocks C. Diffusive Mass Transfer Processes deformation occurs by transfer of material from areas of high stress to areas of low stress 1. Pressure Solution (solution creep) - dissolution of crystal surfaces and fluid-assisted transport of material to new sites of precipitation 2. Coble Creep - diffusion of atoms from crystal surfaces in a dry environment - deformation rates are linked to ease of diffusion and diffusion path lengths 3. High Temperature, Low to Moderate Stress Deformation dominated by diffusive mass transfer • at higher temperatures we can now diffuse atoms or ions out from the interior of a crystal, instead of just the surface • vacancies and interstitials in crystal lattice move in response to applied stress A. Nabarro-Herring Creep - vacancies move toward high stress surfaces while atoms move toward low stress surfaces 4. Deformation by Motion of Crystal Defects A. Crystal Defects 1. point defects 2. grain boundaries
3. dislocations B. Dislocation motion 1. dislocation glide: movement of dislocations above a glide plane 2. dislocation climb: movement of dislocations away (normal) from their original glide plai 5. Deformation by Dislocation Creep E> typically occurs at moderate to high temperatures and pressures A. Dislocation glide results in work hardening because intersecting or interfering glide planes cannot pass one another, resulting in dislocation pile-ups B. Dislocation climb e> facilitates movement of dislocations around one another and thus allows continued strain without work hardening C Dynamic Recrystallization growth of new crystals at the expense of existing, cryst: 1. grain boundary migration unstrained crystals enlarge and consume old, strained crystals 2. subgrain rotation dislocation pile-ups divide crystal into subgrains that are relatively unstrained and surrounded by dislocations 6. Grain Boundary Sliding and sp diffusion across grain boundaries allows grains to realign or reassociate themselves by witching neigh very fast strain rates, 4-5 times faster than Nabarro-Herring Creep assisted by very fine grains and higher temps Th >0.5) insures short diffusion paths and rapid diffusion rates 16-2
16-2 3. dislocations B. Dislocation Motion 1. dislocation glide: movement of dislocations above a glide plane 2. dislocation climb: movement of dislocations away (normal) from their original glide plane 5. Deformation by Dislocation Creep typically occurs at moderate to high temperatures and pressures A. Dislocation Glide results in work hardening because intersecting or interfering glide planes cannot pass one another, resulting in dislocation pile-ups B. Dislocation Climb facilitates movement of dislocations around one another and thus allows continued strain without work hardening C. Dynamic Recrystallization growth of new crystals at the expense of existing, crystals Mechanisms 1. grain boundary migration - unstrained crystals enlarge and consume old, strained crystals 2. subgrain rotation - dislocation pile-ups divide crystal into subgrains that are relatively unstrained and surrounded by dislocations 6. Grain Boundary Sliding and Superplasticity diffusion across grain boundaries allows grains to realign or reassociate themselves by switching neighbors • very fast strain rates, 4-5 times faster than Nabarro-Herring Creep • assisted by very fine grains and higher temps (Th > 0.5) - insures short diffusion paths and rapid diffusion rates