Rheology and deformation mechanisms

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Goal : To understand how different deformation mechanisms control the rheological behavior of rocks Rheology and deformation mechanisms

description

Rheology and deformation mechanisms. Goal : To understand how different deformation mechanisms control the rheological behavior of rocks. Elastic rheologies — e = σ d /E. Griffith cracks. Pre-existing flaw in crystal lattice Accounts for apparent weakness of solids. Crack propagation. - PowerPoint PPT Presentation

Transcript of Rheology and deformation mechanisms

Page 1: Rheology and deformation mechanisms

Goal: To understand how different deformation mechanisms control the rheological behavior of rocks

Rheology and deformation mechanisms

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Elastic rheologies — e = σd/E

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Griffith cracks• Pre-existing flaw in crystal lattice

• Accounts for apparent weakness of solids

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Crack propagation

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Tensile stress concentration

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Failure

1. Cracks coalesce to form fractures

2. Fractures coalesce to form fault zones

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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

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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

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1. Dry diffusion creep

Volume diffusion: movement of atoms through the crystal

Grain-boundary diffusion: movement of atoms around the crystal

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Crystal defects

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Diffusion creep

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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

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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

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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

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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

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Diffusion creepFavored by:• High T

• Very small grain sizes

• Low σd

– Dominant deformation mechanism in the mantle below ~100–150 km

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Material dissolved at high-stress areas and reprecipitated in low-stress areas

2. Dissolution-reprecipitation creep

Reprecipitation

Dissolution

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• Probably diffusion limited

• Also ~linear-viscous rheology

• Viscosity proportional to 1/d3

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• Often involved with metamorphic reactions

• Important deformation mechanism in middle third of continental crust

• Forms dissolution seams (cleavages), veins, and pressure shadows

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Nonlinear rheologies — ė = (σd)n/η

n = stress exponent — typically between 2.4 and 4

Small increases in σd produce large changes in ė

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Dislocation creep

Dislocation: linear flaw in a crystal lattice

Can be shuffled through the crystal

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Dislocation glide

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TEM image of dislocations in olivine

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Dynamic recrystallization driven by dislocations

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Dislocation tangle in olivine

Show recrystallization movie

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Dynamically recrystallized quartz