Download - Rheology and deformation mechanisms
Goal: To understand how different deformation mechanisms control the rheological behavior of rocks
Rheology and deformation mechanisms
Elastic rheologies — e = σd/E
Griffith cracks• Pre-existing flaw in crystal lattice
• Accounts for apparent weakness of solids
Crack propagation
Tensile stress concentration
Failure
1. Cracks coalesce to form fractures
2. Fractures coalesce to form fault zones
Cataclastic flow
• Cataclastic flow: Combination of pervasive fracturing, frictional sliding, and rolling of fragments in fault zone
• Most frictional-brittle faults operate by cataclastic flow
1 2
3 4
Linear-viscous rheologies — ė = σd/η
1. Dry diffusion creep: Diffusion (movement) of atoms in the
crystal lattice accommodated by shuffling of vacancies
2. Dissolution-reprecipitation creep: dissolving material at
high-stress areas and reprecipitating it in low-stress areas
1. Dry diffusion creep
Volume diffusion: movement of atoms through the crystal
Grain-boundary diffusion: movement of atoms around the crystal
Crystal defects
Diffusion creep
Volume diffusionVolume diffusion governed by:
ė = σd x [(αL x VL x μL) x e^(-Q/RT) x (1/d2)]
d = average grain diameter
T = temperature
Constants:αL = constant
VL = lattice volume
μL = lattice diffusion coefficient
R = gas constant
Q = constant
Natural log base, not elongation
ė = σd x [(αL x VL x μL) x e^(-Q/RT) x (1/d2)]
1/viscosity (1/η)
So, ė = σd/η
Therefore, viscosity is proportional to temperature and inversely proportional to (grain size)2
Grain-boundary diffusion
governed by the equation:
ė = σd x (αGB x VL x μGB) x e^(-Q/RT) x (1/d3)
αGB = constant
μGB = lattice diffusion coefficient
ė = σd x [(αGB x VL x μGB) x e^(-Q/RT) x (1/d3)]
1/viscosity (1/η)
So, ė = σd/η
Therefore, viscosity is proportional to temperature and inversely proportional to (grain size)3
Diffusion creepFavored by:• High T
• Very small grain sizes
• Low σd
– Dominant deformation mechanism in the mantle below ~100–150 km
Material dissolved at high-stress areas and reprecipitated in low-stress areas
2. Dissolution-reprecipitation creep
Reprecipitation
Dissolution
• Probably diffusion limited
• Also ~linear-viscous rheology
• Viscosity proportional to 1/d3
• Often involved with metamorphic reactions
• Important deformation mechanism in middle third of continental crust
• Forms dissolution seams (cleavages), veins, and pressure shadows
Nonlinear rheologies — ė = (σd)n/η
n = stress exponent — typically between 2.4 and 4
Small increases in σd produce large changes in ė
Dislocation creep
Dislocation: linear flaw in a crystal lattice
Can be shuffled through the crystal
Dislocation glide
TEM image of dislocations in olivine
Dynamic recrystallization driven by dislocations
Dislocation tangle in olivine
Show recrystallization movie
Dynamically recrystallized quartz