Transcript Week 7b ductile deformation

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 flowCrystal plasticity
kinds of crystal defects
point defects
line defects
crystal plasticity mechanisms
dislocation glide
Dislocation climb
Dislocation climb + glide=creep
twinning
Diffusional mass transfer
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
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)
Beanbag experiment
Relatively shallow crustal
deformation (fault zones)
Ductile deformational processes
Ductile behavior at elevated temperatures
Achieved by motion of crystal defects (error in crystal lattice)
1)Point defects2)Line defects or dislocations
3)Planar defects
Crystal defects
Motion of defects causes permanent strain while the material
remains solid
Ductile deformational processes
Crystal defects
Point defects:
vacancy,
substitution impurity
Interstitial impurity
Vacancies can migrate by exchange
with atoms at neighboring sites–
also called diffusion
Ductile deformational processes
Crystal defects- line defects
Two end-member configurations.
A) Edge dislocation: extra half-plane of atoms in the lattice
Ductile deformational processes
Crystal defects
Two end-member configurations.
A) Screw dislocation: lattice is deformed in a
screw-like fashion
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