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GEO 5/6690 Geodynamics 10 Oct 2014 © A.R. Lowry 2014 Read for Wed 15 Oct: T&S 339-355 Last Time: RHEOLOGY Laboratory studies & mineral physics suggest.

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Presentation on theme: "GEO 5/6690 Geodynamics 10 Oct 2014 © A.R. Lowry 2014 Read for Wed 15 Oct: T&S 339-355 Last Time: RHEOLOGY Laboratory studies & mineral physics suggest."— Presentation transcript:

1 GEO 5/6690 Geodynamics 10 Oct 2014 © A.R. Lowry 2014 Read for Wed 15 Oct: T&S Last Time: RHEOLOGY Laboratory studies & mineral physics suggest two main types of thermally-activated, stress-driven creep (flow) in “solid” rock: Diffusion creep can occur as diffusion of exotic atoms or vacancies through a grain or movement of atoms along grain boundaries (“Coble creep”). The relationship of stress to strain rate is linear, viscoelastic, and sensitive to grain size: Dislocation creep describes movement of dislocations through a lattice, is nonlinear, & dominates unless grain size << 1 mm:

2 Next Journal Article Reading: For Monday Oct 13: Watts & Burov (2003) Lithospheric strength and its relationship to the elastic and seismogenic layer thicknesses. Earth Planet. Sci. Lett. 213(1-2) (Xiaofei will prep discussion materials)

3 Laboratory studies & mineral physics suggest two dominant “flavors” of non-recoverable strain: (1) Linear viscoelastic creep: “Diffusion” where viscosity Here: R = gas constant T = temperature E a = activation energy P = pressure V a = activation volume d = grain diameter D 0 = frequency factor m = 2 in crystal interiors (rock mat’l prop’s)  3 on crystal boundaries

4 Laboratory studies & mineral physics suggest two dominant “flavors” of non-recoverable strain: (2) Nonlinear Viscoelastic: “Dislocation creep” where effective viscosity Here: R = gas constant T = temperature P = pressure E a = activation energy b = dislocation density V a = activation volume n ~ 3 D 0 = frequency factor  = shear modulus (rock mat’l prop’s) Edge dislocation Screw dislocation

5 Laboratory studies of rock strain use roughly the same equations as those derived from first principles in mineral physics, but collapse them to observable constant params depends on: Lithology (pyroxene > olivine > feldspar > quartz) Water fugacity f H2O Temperature T (and to a lesser extent) Strain rate  Grain size d Pressure P. Ideally, we would like to use geophysics to determine each in situ! But it’s not so simple.

6 In Yield Strength Envelopes, we essentially assume a steady-state (i.e., constant strain rate) That assumption is valid for problems in which time- scales are long and stress is ~ constant. Brittle-field (Amonton’s or Byerlee’s law) assumes elastic-plastic constitutive law Ductile assumes Newtonian or non-Newtonian viscous flow

7 Buehler & Shearer, JGR 2010Schutt et al., Geology in prep P n velocity variation Moho temperature from P n & mineral physics Moho temperature T Moho from P n phase:

8 Schutt et al., Geology in prep Moho temperature from P n & mineral physics Wait… What? Temp under ND > NV-UT? (Partly, but not entirely, because the Moho is deeper in the stable part of the continent…)

9 Pyroxene DryWet Feldspar Quartz These temperatures are sufficiently high to ensure lower crustal flow for all likely crustal lithologies, wet or dry… Viscosity is very sensitive also to lithology & water!


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