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Published byCarolyn Rumble Modified about 1 year ago

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

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Elastic rheologies — e = σ d /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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1 2

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3 4

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Linear-viscous rheologies — ė = σ d /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 diffusion Volume diffusion governed by: ė = σ d x [(α L x V L x μ L ) x e^(-Q/RT) x (1/d 2 ) ] d = average grain diameter T = temperature Constants: α L = constant V L = lattice volume μ L = lattice diffusion coefficient R = gas constant Q = constant Natural log base, not elongation

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ė = σ d x [(α L x V L x μ L ) x e^(-Q/RT) x (1/d 2 ) ] 1/viscosity (1/η) So, ė = σ d /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 = σ d x (α GB x V L x μ GB ) x e^(-Q/RT) x (1/d 3 ) α GB = constant μ GB = lattice diffusion coefficient

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ė = σ d x [(α GB x V L x μ GB ) x e^(-Q/RT) x (1/d 3 )] 1/viscosity (1/η) So, ė = σ d /η Therefore, viscosity is proportional to temperature and inversely proportional to (grain size) 3

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Diffusion creep Favored by: High T Very small grain sizes Low σ d –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/d 3

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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 = (σ d ) n /η n = stress exponent — typically between 2.4 and 4 Small increases in σ d 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

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