Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics,

Geodesy, and Field Observations

Roland Burgmann and Georg Dresen

Key Purposes• Concerned with describing material properties through constitutive equations that relate stress and

strain.

• Rheology is fundamental to understanding the evolution and dynamics of Earth and other planets– Ex: localized, episodic deformation (earthquake cycle) in brittle upper crust is coupled to viscous flow in lower

crust and upper mantle. Thus, rheology of viscously deforming rocks at depths are fundamental when trying to understand time-dependent deformation and hazard along fault zones.

• Strength models– Earth's compositional layering and increasing pressure and temperature can result in strong rheological layering.

• Constitutive laws– Time dependent equations relating stress to strain or strain rates.

• Dominant Deformation Mechanisms– Illustrates primary mechanism and appropriate flow law as a function of stress, temperature,

content/composition, grain size.

• Geodetic/Field observations– Postseismic deformation, nontectonic loading events, shear zones.

Summary• Robust constitutive equations exist for major constituents of lower crust and upper mantle that appear consistent with field and

geophysical observations.

• Viscosity estimates based on lab data for rocks deformed at hydrous conditions likely span full range of flow strengths of rocks with more complex mineralogical compositions.

• • Lithospheric strength and rheology differ as a function of makeup, tectonic evolution, and environment of a region. Rheology

strongly varies with depth across continental lithosphere of varying age, temperature, composition.– Rheology varies with age, temperature, composition– Food models of rheology should be considered end member cases

• Backarc and former backarc regions:– Upper mantle is viscously weaker than low crust due to high temperatures and possibly water– Mantle below old cratonic shields is order of magnitude stronger than that found in tectonically active regions

• Viscosity can vary on orders of magnitude based on location

• Strength of crust below seismogenic fault zones weakened through earthquake-cycle effects and other strain weakening processes– Strain weakening up to several orders of magnitude and localization are ubiquitous at all scales

• Deformation mechanisms and rheology depend on:– Thermodynamic conditions– Material parameters– Mechanical state– Can vary over short distances (inside vs outside shear zone) and timescales (earthquake cycle)

Future Issues• Despite direct evidence for localized shear in lower crust and upper mantle, it's not conclusive if low-crust

deformation below active faults is always highly localized, broadly distributed, or transitioning from one to the other with depth.

• Transients in mechanical response due to abrupt or slow changes in loading, structural state, high strain, and others remain to be explored in detail.

• Must further quantify flow parameters for partially molten and polyphase rocks– Changes in rheology and weakening caused by metamorphic reactions are neither well understood nor quantified

• Geodetic explorations of deep rheology form postseismic deformation need to consider deformation early and late in earthquake cycle and couple physically realistic model parameterizations of distributed plastic flow and localized shear.– More postloading studies needed of all types to explore distribution of rheology across continents and plate

boundary zones.

• Field studies should aim to quantitatively evaluate distribution of strain in space and time as fault zones evolve.– Detailed documentation of structural parameters (grain size, variation with shear strain, phase content, temperature)

is needed within and away from shear zones