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Constant stress experiment

elastic. ductile. Constant stress experiment. Constant stress (strain varies). Constant strain (stress varies). Constant stress (strain varies). Constant strain (stress varies). Constant stress (strain varies). Constant strain (stress varies). Constant stress (strain varies).

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Constant stress experiment

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  1. elastic ductile Constant stress experiment

  2. Constant stress (strain varies) Constant strain (stress varies)

  3. Constant stress (strain varies) Constant strain (stress varies)

  4. Constant stress (strain varies) Constant strain (stress varies)

  5. Constant stress (strain varies) Constant strain (stress varies)

  6. Constant stress (strain varies) Constant strain (stress varies)

  7. Boundary condition = Plate motion! elastic ductile Beneath brittle-ductile transition, deformation described by ductile flow law: ε = As σsn deformation occurs throughout crust, therefore yield stress (σy) reached at all depths. FLOW STRESS YIELD STRESS

  8. Constant strain experiment?... (constant plate velocities) Boundary condition = Plate motion! elastic ductile Beneath brittle-ductile transition, deformation described by ductile flow law: ε = As σsn deformation occurs throughout crust, therefore yield stress (σy) reached at all depths. FLOW STRESS σy = σs YIELD STRESS

  9. Constant strain experiment?... (constant plate velocities) Therefore, w decreases until σy = σs v ε = v / w σy Therefore, increase w = decrease ε Undeformed rock σs Shear zone w ε = As σsn σy Therefore, increase w = decrease σs Undeformed rock σy > σs - deformation migrates into shear zone

  10. Constant strain experiment?... (constant plate velocities) Therefore, w decreases until σy = σs v ε = v / w σy Therefore, decrease w = increase ε Undeformed rock σs Shear zone w ε = As σsn σy Therefore, decrease w = increase σs Undeformed rock σy < σs - deformation migrates outwards

  11. Constant strain experiment?... (constant plate velocities) Constant stress experiment... i.e. σy = σs (due to the buffering effect of the yield stress in the surrounding rock) v ε = v / w σy Therefore, decrease w = increase ε Undeformed rock σs Shear zone w ε = As σsn σy Therefore, decrease w = increase σs Undeformed rock σy < σs - deformation migrates outwards

  12. Problems: 1. If plate velocity changes, strain rate (and therefore flow stress) decreases, resulting in w being too large. 2. pre-existing structure might result in the yield stress being relatively low, therefore w can be higher than predicted. 3. Normal and reverse faulting result in temperature changes, leading to microstructural changes which weaken/strengthen the shear zone localizing or distributing the shear. Conclusion: Strength of lithosphere controlled by yield strength of undeformed rock making up the plate (and NOT the weak shear zone).

  13. w = v / Asσyn i.e. width of the shear zone is a function of the plate velocity / ductile flow law Extrapolated from flow laws for feldspar and olivine

  14. Rheology Implications: The microstructures of shear zones evolve differently depending on if they are constant strain rate or constant stress! Dominant mechanism of weakening/localizing in ductile shear zone is grain size reduction (by dynamic recrystallization). Grain size reduction: (= function of stress) 1. Dynamic recrystallization reduces dislocation density (counteracts work hardening) 2. Results in increased grain boundary sliding 3. Results in switch to grain boundary diffusion creep (= grain size sensitive)

  15. Rheology Implications: Constant strain experiments: these grain size reduction mechanisms result in decreasing flow stress which in turn decrease the rate of grain size reduction. (A to C) Grain growth driven by surface energy counteracts grain size reduction, shifting the deformation mechanism to dislocation creep (C to D). Therefore weakening is not permanent! Grain size reduction: (= function of stress) 1. Dynamic recrystallization reduces dislocation density (counteracts work hardening) 2. Results in increased grain boundary sliding 3. Results in switch to grain boundary diffusion creep (= grain size sensitive)

  16. Rheology Implications: Constant stress experiments: flow stress is maintained. Therefore, the rate of grain size reduction does NOT decrease, and there is no stress drop. (A to B) Grain growth driven by surface energy is inhibited by the faster grain boundary migration. Therefore, no further evolution of grain size. Therefore grain size reduction = dominant cause of strain localization in the lithosphere! Grain size reduction: (= function of stress) 1. Dynamic recrystallization reduces dislocation density (counteracts work hardening) 2. Results in increased grain boundary sliding 3. Results in switch to grain boundary diffusion creep (= grain size sensitive)

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