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Rheology of the Earth

Rheology of the Earth. Schedule. Rheology Viscous, elastic & plastic Deformation maps and “Christmas tree’s” for mantle & lithosphere Constrains of rheology of the Earth from geology & geophysics. Rheology: what is it?. The study of how material flows Examples. Viscous creep law.

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Rheology of the Earth

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  1. Rheology of the Earth

  2. Schedule • Rheology • Viscous, elastic & plastic • Deformation maps and “Christmas tree’s” for mantle & lithosphere • Constrains of rheology of the Earth from geology & geophysics

  3. Rheology: what is it? The study of how material flows Examples

  4. Viscous creep law • Experimental data: • Viscosity is strongly dependent on pressuretemperature, stress (strain-rate),grain size, water, melt, &mineralogy…

  5. Byerlees law Empirical fit to Mohr Coulomb

  6. Strength of the Lithosphere and Mantle (Kohlstedt et al., 1995) Strength curves for different materials: lithosphere

  7. Difference between compression/extension Burov Treatise on Geophysics V. 6 (2007) -> Difference in come from the dependence of Byerlee’s law on normal stress. Compression results in larger normal stress (‘tectonic loading’)

  8. Constraints on rheology from geophysics

  9. Post-Glacial Rebound (PGR)

  10. Post-Glacial Rebound (PGR) • Rate of rebound: • sensitive to absolute viscosity. • Depends on: • ice-load size/shape, sea-level measurements & unloading history. • lateral variations in elastic plate properties. From: http://www.pgc.nrcan.gc.ca/geodyn/ docs/rebound/glacial.html

  11. PGR • Haskell (1935): 1021 Pas Kaufman & Lambeck, PEPI (2000)

  12. Geoid From M. Billen

  13. Geoid Range +/- 120 meters

  14. Geoid • Observations from seismic studies • Long wave length geoid LOWS correlate with seismically FAST regions, ie cold dense regions • Long wave length geoid HIGHS correlate with seismically SLOW regions, ie.hot buoyant regions. • This is initially counter intuitive

  15. Geoid Sensitive to radial and lateral viscosity structure. Layer 1 Layer 2 From: M. Billen (MYRES)

  16. Geoid: Very long wavelength structure explained by lower mantle structure. Jump or increase in viscosity from upper to lower mantle (+/- factor 30). “Robust” Constraints on Viscosity Structure Observed Predicted From: Hager & Richards, phil trans 1989, (fig 1, 5a)

  17. Plate motions • Purely radial viscosity structure • poloidal motion (divergence/ convergence) . • How to use in modelling? • Impose as boundary conditions. • Predict from model (defined plate regions). Observed Predicted From: Conrad &Lithgow-Bertelloni, Science 2003

  18. Plate motions & mantle viscosity structure Becker (2006) Assume free slip boundary at CMB and surface Choose flow laws Invert for radial viscosity structure Invert for rheolgy

  19. Plate motions & mantle viscosity structure

  20. Observables: summary • Geoid • Plate motion • Postglacial rebound • Lab experiments, but large extrapolation involved.

  21. Rheology: • Viscous, elastic, plastic & combinations • Viscous: diffusion creep & powerlaw creep. • Plastic: stress-limiting mechanism. • Elastic: recoverable. • Mostly based on lab experiments • Difficulties of extrapolating • YSE & consequences of rheology for deformation of the lithosphere • Geophysical constraints • Geoid, plate motion, PGR

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