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

Bürgmann and Dresen, 2008

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

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

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

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

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

1. Laboratory Experiments Bürgmann and Dresen, 2008

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

2. Geodetic Inferences Bürgmann and Dresen, 2008

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

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

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

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

Nontectonic Loading Events 3D GPS velocities → estimate upper-mantle viscosities of 5-10 x 1020 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

3. Field Observations Bürgmann and Dresen, 2008

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

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

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

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

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