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
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