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
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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

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

  • Elastic rheologies e = d/E

  • Griffith cracksPre-existing flaw in crystal latticeAccounts for apparent weakness of solids

  • Crack propagation

  • Tensile stress concentration

  • Failure1. Cracks coalesce to form fractures 2. Fractures coalesce to form fault zones

  • Cataclastic flowCataclastic flow: Combination of pervasive fracturing, frictional sliding, and rolling of fragments in fault zoneMost frictional-brittle faults operate by cataclastic flow

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  • Linear-viscous rheologies = d/ Dry diffusion creep: Diffusion (movement) of atoms in the crystal lattice accommodated by shuffling of vacancies Dissolution-reprecipitation creep: dissolving material at high-stress areas and reprecipitating it in low-stress areas

  • 1. Dry diffusion creepVolume 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: = d x [(L x VL x L) x e^(-Q/RT) x (1/d2)]d = average grain diameterT = temperatureConstants:L = constantVL = lattice volumeL = lattice diffusion coefficientR = gas constantQ = constant

    Natural log base, not elongation

  • = d x [(L x VL x L) x e^(-Q/RT) x (1/d2)]

    1/viscosity (1/)So, = d/Therefore, viscosity is proportional to temperature and inversely proportional to (grain size)2

  • Grain-boundary diffusiongoverned by the equation: = d x (GB x VL x GB) x e^(-Q/RT) x (1/d3)GB = constantGB = lattice diffusion coefficient

  • = d x [(GB x VL x GB) x e^(-Q/RT) x (1/d3)]

    1/viscosity (1/)So, = d/Therefore, viscosity is proportional to temperature and inversely proportional to (grain size)3

  • Diffusion creepFavored by:High TVery small grain sizesLow d

    Dominant deformation mechanism in the mantle below ~100150 km

  • 2. Dissolution-reprecipitation creepMaterial dissolved at high-stress areas and reprecipitated in low-stress areasReprecipitationDissolution

  • Probably diffusion limitedAlso ~linear-viscous rheologyViscosity proportional to 1/d3

  • Often involved with metamorphic reactionsImportant deformation mechanism in middle third of continental crustForms dissolution seams (cleavages), veins, and pressure shadows

  • Nonlinear rheologies = (d)n/ n = stress exponent typically between 2.4 and 4Small increases in d produce large changes in

  • Dislocation creepDislocation: linear flaw in a crystal latticeCan be shuffled through the crystal

  • Dislocation glide

  • TEM image of dislocations in olivine

  • Dynamic recrystallization driven by dislocations

  • Dislocation tangle in olivineShow recrystallization movie

  • Dynamically recrystallized quartz