Roland Burgmann and Georg Dresen

1. Rheology of the lower crust and upper mantle: Evidence from rock mechanics, geodesy, and field observations Roland Burgmann and Georg Dresen

2. Rheology: The seed of life!

3. WTF?!

4. Which flavor are you? • PB&J (European version: Jelly Sandwich): • Increasing P and T as well as compositional layering = rheological layering • Pressure-dependent increase of frictional strength with depth bound by thermally activated creep processes • Reduces viscous strength with increasing T and depth • Week middle and lower crust bounded by strong upper crust and mantle lithosphere • Long-term strength of tectonic plates lies in the lithospheric mantle

5. Which flavor are you? • Crème Brulee: • Strength of continental lithosphere entirely in the crust • Upper mantle weaker due to high T and H2O • Close correlation between thickness of seismogenic layer and estimates of the elastic thickness of the continental lithosphere • “Regional estimates of continental elastic thickness from gravity data vary significantly and do not necessarily preclude a strong mantle layer” (from Burov and Watts, 2006)

6. Which flavor are you? • Banana Split: • Reduced strength along plate boundaries • Weakening processes include: thermal, fluid, strain-rate • San Andreas and other mature fault zones interpreted by some as frictionally very weak • Stress orientations, heat flow, and seismic observations • In Viscous regime shear heating, grain-size reduction, dynamic recrystallization, chemical alterations and phase changes, and development of rock fabrics lead to weakened shear zones

7. Byerlee’s Law • Emperical solution to the mohr-coulomb failure criterion • τ= S0 + μ(σn − Pf) • τ = 0.85σn for stresses < 200 MPa • τ = 50 + 0.6σn for stresses > 200 MPa • Linear function of shear stress over normal stress (Wikipedia)

8. Discussion? • Likely to vary with the tectonic environment, lithology, temperature, availability of fluid, time, etc. • “….varied cuisine of continental rheology” • When did our concern become focused on only continental rheology?

9. Review: Maxwell vs. Kelvin solid

10. Deformation Mechanisms • Constitutive behavior depends on: • Phase content • Chemical composition • Thermodynamic variables • Diffusion creep: Linear • Dislocation creep: Power-law • Variables and parameters • T • Stress • Melt content • Water content • Grain size

11. Rate- and State-dependent friction laws • Velocity-weakening • Friction decreases with sliding velocity • Fault rupture • Velocity-strengthening • Friction increases or remains stable with sliding velocity • Aseismic slip

12. Lab experiments

13. Additional Comments • Silicate rocks deformed under hydrous conditions are significantly weaker than at anhydrous conditions, in case you weren’t trackin’ • Deformation of lower-crustal and upper-mantle rocks may involve varying amounts of partial melt • Experimental studies of polyphase rocks are still few

14. Geodesy • Postseismic deformation • 1906 San Fracisco earthquake • Initiated use of geodesy to infer rheology at depth • Recognition of elastic rebound • Models that incorporate weak vertical shear zones in the crust beneath major faults • Relaxation of a deep lower-crustal or mantle layer with effective viscosities >= 1020 Pa s

15. Geodesy • Relaxation processes? • Distributed viscous shear and localized aseismic slip can produce same pattern of post-seismic surface deformation for a 2D, infinite length, strike slip EQ • Distribution of deep relaxation not as ambiguous when considering 3D post-seismic deformation w/ finite length ruptures

16. Geodesy • Landers and Hector Mine • Denali • Manyi • Hebken Lake • Andaman/Sumatran

17. Geodesy: Denali

18. Geodesy: Interseismic Events

19. Geodesy: Nontectonic Loading Events • Glaciers and Lakes • Limits noise due to afterslip, etc. (except maybe in the Basin and Range…..) • Lake Mead • Lake Bonneville • Glaciers

20. Geodesy

21. Geodesy: Discussion • Lake vs. Glacial • Rate of gravity change and upper-mantle viscosity? • Lower effective mantle viscosities under thinner lithospheric lids than those found in cratonic shields (Cascadia) • Current rates of uplift

22. The Field! • The field?

23. The Field! • Geophysical imaging of fault zones • Limitations in imaging deep sections of active faults • Spacial resolution limitations at great depths • Marlborough fault • Smoothly varying Moho depths and pervasive anisotropy below 15 km demonstrate broad distribution of deformation • Anisotropy imaged in mantle lithosphere • Suggests distributed shear • Homogenous shearing? Below Alpine fault in NZ (Molnar et al., 1999) • Preferred lattice orientation in upper 200 km of manlte suggests deformation to that depth likely dislocation creep

24. The Field! • Exhumed Fault Zones • Significant reduction of grain size toward high shear strains = intercalated fine-grained mylonites • Intercalate=Insert (something) between layers in a crystal lattice, geological formation, or other structure. (online Webster’s dictionary) • Processes: dynamic recrystallization, cataclasis, mineral reactions producing fine-grained and weaker reaction products

25. The Field! • Crustal Roots of Active Faults • Alpine Fault in NZ • Extremely localized shear zone widths, less than 5 km • Shear strain increased by more than an order of magnitude toward the trace of the active fault • Mylonites cut by brittle faults and pseudotachylytes reflecting rise of the shear zone through the brittle-ductile transition zone

26. The Field! • Fossil shear zones • Width of shear zone appears to narrow with decreasing T and depth • Deep crustal beds have recorded mylonites up to 30 km in width • Recent estimates suggest viscosity reduction in shear zones up to factor of 100 • Shear zones observed in mantle xenoliths and such suggest shear in upper mantle • Mantle shear zone width varies from several km to a few mm • Often accomodated by anastomosing networks

27. The Field! • Fossil shear zones

28. Grain Size • Critical parameter for strength and deformation mechanisms • Grain sizes in mylonites range from 5 to 300 μm • Generally, at decreasing grain size, diffusion creep becomes more important

29. The Field! • Discussion:

30. Reconciling the Data • PB&J • Geodetic studies suggest upper mantle (or asthenospheric mantle?) beneath thin mantle lid is actually weaker in many tectonically active regions • I thought it was widely understood that the asthenosphere is a weak zone • Trace contents of water substantially weaken lower-crustal rocks, but do not generate partial melt in gabbroic rocks • Hypothesized models for very weak lower-crustal channels of hydrolytically weakened, quartz-rich, and partially melted materials

31. Reconciling the Data • Crème Brulee • In regions lacking strong lithospheric mantle roots, lithospheric strength dominated by crust • Banana split • Mature plate boundary fault zones weaken as they evolve at all depth levels • Mylonites

32. Questions?