Friday 12:00 Geology Seminar Dr. Lucy Flesch, Purdue University
Integration of Plate Boundary Observatory and USArray Data to Quantify the Forces Driving Deformation in the Western United States Nisqually Earthquake, Feb 28, 2001
6.8 Mw 52 km deep No deaths ~400 injuries Fault strength paradox:
San Andreas Fault and Pore Fluid Pressure Outline: Ductile Deformation Three main mechanisms Cataclastic flow- Crystal plasticity kinds of crystal defects point defects line defects crystal plasticity mechanisms dislocation glide Dislocation climb Dislocation climb + glide=creep twinning Diffusional mass transfer Solid State Mass diffusion Grain Boundary mass diffusion Ductile deformational processes
Introduction: how can rocks bend, distort, or flow while remaining a solid? Non-recoverable deformation versus elastic deformation Ductile behavior weve used the words viscous and plastic to describe the deformation- now well talk about the actual physical processes Three mechanisms: 1) Catalclastic flow 2) Crystal plasticity 3) Diffusional mass transfer Which process dominates controlled by: temperature stress strain rate grain size composition fluid content FYI, what is considered high-temp behavior for one mineral is low-temp. behavior for another mineral. Normalized paramater, homologous temperature, Th Th = T/Tm Different rocks/minerals behave ductily at different temperatures: Homologous temperature: Th=T/Tm Low temperature~ Th300 for quartz rich rocks >500 feldspar, olivine Ductile deformational processes
Crystal Plasticity: migration of crystal dislocations causes permanent deformation Dislocations (line defects) can move by glide, climb or cross slip Glide + climb/cross slip is often calleddislocation creep Another crystal-plastic behavior is twinning Ductile deformational processes
Crystal Plasticity: migration of crystal dislocations causes permanent deformation Dislocations (line defects) can move by glide, climb or cross slip creep Another crystal-plastic behavior is twinning Ductile deformational processes
Crystal Plasticity: migration of crystal dislocations causes permanent deformation twinning twins that develop during growth of mineral (Growth twins), have little to nothing to say about conditions of deformation Ductile deformational processes
Crystal Plasticity: migration of crystal dislocations causes permanent deformation twinning Mechanical twins: twins formed in response to an applied stress. Common in calcite Ductile deformational processes
Crystal Plasticity: migration of crystal dislocations causes permanent deformation twinning Startingmineral Apply differentialstress Dislocation boundary forms Twinningplane Partial dislocations glide, form twin Ductile deformational processes
Crystal Plasticity: migration of crystal dislocations causes permanent deformation twinning Mechanical twinning: crystal plastic process that involves glide of partial dislocation- atoms move a fraction of a lattice distance Favored under faster strain rates, lower temperatures Ductile deformational processes
Diffusional mass transfer: occurs when an atom (or point defect) migrates through a crystal Easier for atoms to move around at higher temperatures => Diffusion rate faster at higher temperatures D is diffusivity D0 is a diffusion constant for a given material (i.e., calcite, quartz, etc) E* is the activation energy (kJ/mol) R is the gas constant (8.31 J/mol*K) T is absolute temperature (in K) Ductile deformational processes
Diffusional mass transfer: occurs when an atom (or point defect) migrates through a crystal Solid State Diffusion: volume diffusion, grain-boundary diffusion Grains change shape to adjust to stress field Outline: Ductile Deformation Three main mechanisms Cataclastic flow- Crystal plasticity crystal plasticity mechanisms dislocation glid Dislocation climb Dislocation climb + glide=creep twinning Diffusional mass transfer solid state mass transfer pressure solution mass transfer Outline: Ductile Deformation Three main mechanisms Cataclastic flow- Crystal plasticity kinds of crystal defects point defects line defects crystal plasticity mechanisms dislocation glide Dislocation climb Dislocation climb + glide=creep twinning Diffusional mass transfer solid state mass transfer pressure solution mass transfer Constitutive Equations (flow laws) Mechanism Maps Not sects 9.7, 9.8, 9.9just overview of 9.10 Ductile deformational processes
Diffusional mass transfer: occurs when an atom (or point defect) migrates through a crystal Pressure Solution: At areas of high stress, grains dissolve into fluid film, then migrate to region of low stress, and recrystalize Occurs at relatively low temperatures => Important deformation mechanism in the upper crust Pressure Solution Video Ductile deformational processes
Diffusional mass transfer: occurs when an atom (or point defect) migrates through a crystal Pressure Solution Stylolites (pressure solution seams) in limestone of Mississippian age, exposed on the side of a rounded boulder in Hyalite Canyon, Gallatin Range, Montana. These stylolites, like most, are bedding-parallel, and thus most likely formed due to the weight of the overlying rock. Calcite, the dominant mineral, goes into solution under pressure, and insoluble material, like organic matter and clay, accumulates along the dissolution surface, producing a dark, wiggly line. Here, multiple stylolites have converged and overprinted one another, resulting in a mutli-level oscilloscope look. Ductile deformational processes
Constitutive Equations or Flow laws Relating strain (or strain rate) to stress Strain rate Stress function material constant Activation energy Gas constant Temperature is strain rate (s-1) A is a material constant E*is the activation energy R is the gas constant T is the absolute temperature is a function of differential stress Remember the diffusion equation? A E R T Ductile deformational processes
Constitutive Equations or Flow laws Relating strain (or strain rate) to stress For dislocation glide, the function of stress is exponential =exp(sd) = exp(sd) For dislocation glide and climb (creep), the function of stress is raised to the power n =(sd)n = sdn For diffusion, the stress function is stress and the grain size (d) = = Deformation Mechanisms
Important relations Normalized stress (normalized to shear modulus of the material versus normalized temperature (normalized to absolute melting temperature of the material) dislocation glide: exponential dislocation creep, power law An area of the crystal that has slipped relative to the rest of the crystal. diffusion, grain size (d) Deformation Mechanisms
Important relations Differential stress versus Temperature An area of the crystal that has slipped relative to the rest of the crystal. Deformation Mechanisms
Crystalline structures and defects within rocks can deform by a variety of deformation mechanisms.The mechanism or combination of mechanisms in operation depends on a number of factors: Mineralogy & grain size Temperature Confining and fluid pressure Differential stress (s1 - s3) Strain rate In most polymineralic rocks, a number of different defm. mechanisms will be at work simultaneously. If conditions change during the deformation so will the mechanisms. The Main Deformation Mechanisms
5 General Catagories: 1) Microfracturing, cataclastic flow, and frictional sliding. 2) Mechanical twinning and kinking. 3) Diffusion creep. 4) Dissolution creep. 5) Dislocation creep. Deformation Mechanism Map
Cataclasis Dissolution creep Dislocation creep Diffusion creep Pressure solution Each of these mechanisms can be dominant in the creep of rocks, depending on the temperature and differential stress conditions. Depth / Temperature