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 – we’ve used the words viscous and plastic to describe the deformation- now we’ll 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~ Th<0.3 medium temperature~ 0.3<Th<0.7 High temperature~ Th>0.7

Ductile deformational processes Catalclastic flow Cataclastic flow: rock fractured into smaller particles that slide/flow past one another Large grain microfracture at grain boundary scale or within individual grains Remains cohesive (vs gouge or breccia) Relatively shallow crustal deformation (fault zones) 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 Beanbag experiment

Ductile deformational processes Ductile behavior at elevated temperatures Achieved by motion of crystal defects (error in crystal lattice) Point defects- Line defects or dislocations Planar defects Crystal defects Vacancies – unoccupied siter in the crystal lattice and replaced by different atom (b) (c) Or by interstitails, atom at a non-lattice site Vacancies can migrate by exchange with atoms at neighboring sites (d) – also called diffusion Motion of defects causes permanent strain while the material remains solid

Ductile deformational processes Crystal defects Point defects: vacancy, substitution impurity Interstitial impurity Motion of defects gives rise to permanent strain with the material losing cohesion (no fracturing) Vacancies can migrate by exchange with atoms at neighboring sites– also called diffusion

Ductile deformational processes Crystal defects- line defects Two end-member configurations. Edge dislocation: extra half-plane of atoms in the lattice An area of the crystal that has slipped relative to the rest of the crystal.

Ductile deformational processes Crystal defects Two end-member configurations. A) Screw dislocation: lattice is deformed in a screw-like fashion An area of the crystal that has slipped relative to the rest of the crystal.

Ductile deformational processes Crystal defects Burgers vector b: The vector that represents the magnitude and direction of the lattice distortion

Ductile deformational processes Crystal defects Burgers vector b: The vector that represents the magnitude and direction of the lattice distortion Magnitude of Burgers vector commonly on the order of nanometers (1 x 10-9 m)

Ductile deformational processes Crystal defects Mixed dislocations: combination of edge and screw Defects cause internal stress, can affect the way the mineral responds to external stress:

Ductile deformational processes Crystal defects and stress

Ductile deformational processes Crystal defects

P.S. #4 coming soon… Mid Term this Thursday, review sheet on course website Folds and Stereonets Lab- solutions of contoured fold data on east wall

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)

Crystal defect movies Bubble raft example

Ductile deformational processes Crystal Plasticity: The migration of crystal dislocations causes permanent deformation Once activation energy is achieved, dislocations can migrate Energy sources: distortion of lattice due to dislocation heat differential stress

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 (creep) Another crystal-plastic behavior is twinning

Ductile deformational processes Crystal Plasticity: migration of crystal dislocations causes permanent deformation Dislocations can move by glide,

Ductile deformational processes Crystal Plasticity: migration of crystal dislocations causes permanent deformation Dislocations can move by glide, The glide plane contains the Burgers vector and the dislocation line Edge dislocation: burger vector and dislocation line are perpendicular: Therefore, any edge dislocation has only one plane orientation to glide on

Ductile deformational processes Crystal Plasticity: migration of crystal dislocations causes permanent deformation Dislocations can move by glide, The glide plane contains the Burgers vector and the dislocation line Screw dislocation: Burgers vector and dislocation line are parallel: Therefore, any screw dislocation has many plane orientations to glide on

Ductile deformational processes Crystal Plasticity: migration of crystal dislocations causes permanent deformation Dislocations can move by glide, Dislocations glide to the edge of the grain, can produce stair-step structure called : slip bands

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 (creep) Another crystal-plastic behavior is twinning

Ductile deformational processes Crystal Plasticity: migration of crystal dislocations causes permanent deformation Dislocation by climb or cross slip, Sometimes glide planes are blocked by point defects. Edge Dislocation: can’t continue on that plane, so dislocation climbs to new glide plane Requires significant energy for edge dislocation

Ductile deformational processes Crystal Plasticity: migration of crystal dislocations causes permanent deformation Dislocation by climb or cross slip, Requires significant energy for edge dislocation Since screw dislocations have many possible glide planes, easily cross-slip to another plane

Ductile deformational processes Crystal Plasticity: migration of crystal dislocations causes permanent deformation Dislocation by climb or cross slip, Requires significant energy for edge dislocation Since screw dislocations have many possible glide planes, easily cross-slip to another plane Both Climb and Cross slip do need extra energy => typicaly occur at deeper (hotter) levels in the earth. >300° 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 called dislocation 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 Starting mineral Apply differential stress Dislocation boundary forms Twinning plane 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, 9.9 just 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