Lecture 13: Deformation mechanisms 1. Distribution of deformation in the crust sp divide the crust into deformation regimes based on temperature and depth A Upper Crust (0-50 km) deformation localized along narrow zones or faults characterize d by stick-slip seismic or creep behavior B l o. aults may exhibit both types of behavior(even on same fault) ower Crust(40-70 km) diffuse deformation spread over larger volumes dominated by ductile flow processes Ct trong influence of oe, mineralogic changes, temperature and fluids eformation Mechanism Maps sp show dominant modes of deformation under given stresses and temperatur can determine experimentally for monomineralic polycrystalline materials difficult or impossible to determine for polymineralic materials 2. Low Temperature Deformation p occurs below homologous temperatures, Th=0.3(Th=T/Tm) for framework minerals A. Brittle mechanisms and Particulate Flow 1. frictional siding slip occurs when Coulomb criterion for sliding is met affected by fluid pressure 2. gramular ow: rolling and sliding of particles past and over one another 3. cataclastic ow continuous brittle fracture and comminution of grains with some actional sliding or rolling may dramatically affect porosity (e.g. deformation bands) B. Twin gliding E> 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 can occur in many minerals according to different twin laws and at different P-t conditions 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 p 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 occurs at higher temps than pressure solution deformation rates are linked to ease of diffusion and diffusion path lengths 3. High Temperature, Low to Moderate Stress Deformation sp 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 13-1
Lecture 13: Deformation Mechanisms 13-1 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 volumes - 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 • can determine experimentally for monomineralic polycrystalline materials • difficult or impossible to determine for polymineralic materials 2. Low Temperature Deformation occurs below homologous temperatures, Th = 0.3 (Th = T/Tm ) for framework minerals A. Brittle Mechanisms and Particulate Flow 1. frictional sliding: slip occurs when Coulomb criterion for sliding is met - affected by fluid pressure 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 - may dramatically affect porosity (e.g. deformation bands) 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 • can occur in many minerals according to different twin laws and at different P-T conditions • 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 - occurs at higher temps than pressure solution - 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
vacancies move toward high stress surfaces while atoms move toward low stress surfaces grain shape change continually increases diffusion path lengths and results in slowing down of creep defo 4. Deformation by motion of Crystal Defects A Crystal Defects 1. point defects (interstitials, vacancies, substitutional 2. grain boundaries 3. dislocations (edge and screw types) extra half-plane of atoms in the crystal lattice B. Dislocation motion E> crystal defects move in response to applied stress 1. dislocation glide movement of dislocations above a glide plane 2. dislocation climb movement of dislocations away(normal) from their original glide plane dislocations glide and climb about, may entangle or annihilate one another, increasing or decreasing the overall strain energy of a crystal or polycrystalline material this affects the ease of continued deformation 5. Deformation by Dislocation Creep s> typically occurs at moderate to high temperatures and pressures sp may occur at low temps with very high stresses A. Dislocation glide p results in work hardening because intersecting or interfering glide planes cannot pass one another, resulting in dislocation pile creep under constant stress eventually stops B. Dislocation climb p facilitates movement of dislocations around one another and thus allows continued strain without work hardening occurs above T=0.5 responsible for"steady-state"creep C. Dynah of new crystals at the expense of existing crystals mic recrystallization 今 Mechanisms 1. grain boundary migration unstrained crystals enlarge and consume old, strained crystals results in irregular, serrated grain boundaries separating strained and unstrained grains 2. subgrain rotation dislocation pile-ups divide crystal into subgrains that are relatively unstrained and surrounded by dislocations subgrains rotate until crystallographically compatible and then can combine or be assimilated by unstrained grains results in systematic variation in optical continuity of subgrains from interior to the edges of parent grains 13-2
13-2 A. Nabarro-Herring Creep - vacancies move toward high stress surfaces while atoms move toward low stress surfaces - grain shape change continually increases diffusion path lengths and results in slowing down of creep deformation 4. Deformation by Motion of Crystal Defects A. Crystal Defects 1. point defects (interstitials, vacancies, substitutionals) 2. grain boundaries 3. dislocations (edge and screw types) - extra half-plane of atoms in the crystal lattice B. Dislocation Motion crystal defects move in response to applied stress 1. dislocation glide: movement of dislocations above a glide plane 2. dislocation climb: movement of dislocations away (normal) from their original glide plane • dislocations glide and climb about, may entangle or annihilate one another, increasing or decreasing the overall strain energy of a crystal or polycrystalline material - this affects the ease of continued deformation 5. Deformation by Dislocation Creep typically occurs at moderate to high temperatures and pressures may occur at low temps with very high stresses A. Dislocation Glide results in work hardening because intersecting or interfering glide planes cannot pass one another, resulting in dislocation pile-ups • creep under constant stress eventually stops B. Dislocation Climb facilitates movement of dislocations around one another and thus allows continued strain without work hardening • occurs above Th = 0.5 • responsible for “steady-state” creep 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 - results in irregular, serrated grain boundaries separating strained and unstrained grains 2. subgrain rotation - dislocation pile-ups divide crystal into subgrains that are relatively unstrained and surrounded by dislocations - subgrains rotate until crystallographically compatible and then can combine or be assimilated by unstrained grains - results in systematic variation in optical continuity of subgrains from interior to the edges of parent grains
6. Grain Boundary Sliding and Superplasticity p diffusion across grain boundaries allows grains to realign or reassociate themselves by switching neighbors grain shapes are retained very short diffusion path lengths very fast strain rates, 4-5 times faster than Nabarro-Herring Creep assisted by very fine grains and higher temps Th>0.5) these characteristics insure short diffusion paths and rapid diffusion rates 13-3
13-3 6. Grain Boundary Sliding and Superplasticity diffusion across grain boundaries allows grains to realign or reassociate themselves by switching neighbors • grain shapes are retained • very short diffusion path lengths • very fast strain rates, 4-5 times faster than Nabarro-Herring Creep • assisted by very fine grains and higher temps (Th > 0.5) - these characteristics insure short diffusion paths and rapid diffusion rates
