1. Roland Bürgmann and Georg Dresen
Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics, Geodesy, and Field Observations
Roland Bürgmann and Georg Dresen

2. Bürgmann and Dresen, 2008

3. First-order models of lithospheric strength
Bürgmann and Dresen, 2008

4. First-order models of lithospheric strength
Weak middle and lower crust bounded by strong upper crust and strong mantle
↑ μ with pressure and depth in upper crust – Byerlee’s law
↓ μ with pressure and depth in middle and lower crust – thermally activated creep processes – reduce viscous strength
Range of deformation mechanisms across Moho (T- dependent)
↑ μ with pressure and depth – mafic, higher MP, higher viscosities
Jelly sandwich model
Bürgmann and Dresen, 2008

5. First-order models of lithospheric strength
Strong lower crust – dry and brittle,
feldspar and pyroxene-dominated,
trace H2O can reduce viscosity but insufficient to generate substantial partial melt
Weak upper mantle – high temp. and fluids
Crème brûlée model
Bürgmann and Dresen, 2008

6. First-order models of lithospheric strength
Lateral strength reduction of the lithosphere along plate boundaries due to weakening processes
Frictionally weak mature faults
( μ < 0.2) – paradox
Shear heating, grain-size reduction, dynamic crystallization, chemical alteration etc.
Deformation governed by discrete, weakened shear zones OR by bulk rheological properties of viscously deforming lower lithosphere?
Banana split model
Bürgmann and Dresen, 2008

7. Rheology: constitutive laws
Linear elastic (Hooke solid)
Linear viscous (Newtonian fluid)
Immediate elastic → Newtonian (Maxwell fluid)
Viscously damped elastic response (Kelvin solid)
Biviscous, more than one relaxation time (Burger body)
Bürgmann and Dresen, 2008

8. 1. Laboratory Experiments
Bürgmann and Dresen, 2008

9. Rheological behavior of materials representative of the lower crust and upper mantle
↑ differential stress and ↑ grain size, dislocation creep dominant
For grain size < 200μm: diffusion-controlled creep (exception of qtz)
For qtz: need much finer grain size to accommodate diffusion creep (<10 μm), otherwise dislocation creep
Silicate rocks deformed under hydrous conditions are significantly weaker than at anhydrous conditions
Deformation mechanism maps for wet rheologies
Bürgmann and Dresen, 2008

10. 2. Geodetic Inferences
Bürgmann and Dresen, 2008

11. 2002 Denali earthquake
Use changes in rate of observed surface displacements (a)
Considered power law exponents from 1 to 5
n = 1 : linear viscous mantle cannot explain fast early displacement rates at the surface that rapidly decays with time
n > 1 : decrease in effective viscosity and an increase of strain rate as differential stress increases ( ̴̴ 3.5)
Best fit → Power-law (b)
Bürgmann and Dresen, 2008

12. 2002 Denali earthquake
Post seismic deformation within the lower crust and upper mantle due to viscoelastic relaxation
Consistent with lab experiments and field studies (suggest dislocation creep in lower crust and upper mantle)
Bürgmann and Dresen, 2008

13. 2002 Denali earthquake
Immediately after earthquake, upper mantle behaves as a relatively weak region concentrating strength in the middle and upper crust (c) → Crème brûlée model
Long term strength does not recover to crustal levels
Bürgmann and Dresen, 2008

14. Nontectonic Loading Events
Earthquakes occur along active fault zones – anomalous structure, associated with sudden stress-change events
Advantage of studying rheology away from active faults
Examine glacial retreat, fluctuations of reservoirs instead to determine mechanical properties of the underlying mantle
Isostatic rebound of late-Pleistocene shorelines of paleolakes in the eastern and western Basin and Range province – studies suggest both regions underlain by low-viscosity mantle
Bürgmann and Dresen, 2008

15. Nontectonic Loading Events
3D GPS velocities → estimate upper-mantle viscosities of 5-10 x Pa
GRACE satellite → Rate of gravity change best fit by an upper-mantle viscosity of 8 x 1020 Pa
High viscosities associated with the uppermost mantle below cratonic shields
Bürgmann and Dresen, 2008

16. 3. Field Observations
Bürgmann and Dresen, 2008

17. Field observations
Significant decrease in grain size towards high shear strains resulting in fine-grained ultramylonite layers
In general, cataclasis, dynamic recrystallization, mineral reactions → fine-grained and weaker reaction products
Mylonites (localized downward extension of the Alpine fault) intersected by pseudotachylytes, reflecting rise of the shear zone through the brittle-ductile transition zone → shear localization in the lower crust
Bürgmann and Dresen, 2008

18. Satellite images of mylonite shear zones
Large-scale shear zones: anastomosing networks of dense mylonite layers separating less deformed material
Kinematic indicators (sigmoidal, deltoid patterns etc.) provide insight into rheology
Grain size reduction → strain localization → reduce viscosity of shear zones
Bürgmann and Dresen, 2008

19. Evidence of shear localization in mantle
Mylonites found in dredged mid-ocean-ridge peridotites → common in the uppermost mantle (low temp. high strength)
Presence of feldspar clasts in fine grained matrix of dominantly olivine and pyroxene → mantle mylonites weaker than coarse grained feldspar-bearing crustal rocks
Weakening observed in mantle shear zones: up to 4 orders of magnitude than the surrounding host rock
Bürgmann and Dresen, 2008

20. Paleostress estimates from mylonite shear zones transecting lower crust (a) and upper mantle (b)
Extrapolated lab data for dislocation creep of rocks at hydrous conditions
Bürgmann and Dresen, 2008

21. MAJOR CONCLUSIONS
Strength and rheology varies with depth and laterally
Mature faults are weak: varying degrees of localization
Deformation in the uppermost mantle can also localize
Silicates deformed under hydrous conditions – hydrolytic weakening
Deformation mechanisms can vary over short spatial and temporal scales