Initiation and preservation of localized deformation in the

mantle

Phil SkemerWashington University in St. Louis

Structural Geology and Tectonics ForumJune 16, 2014With contributions from:

Rolf Bruijn, Jolien Linckens, Jessica Warren, Lars Hansen, Greg Hirth, Peter Kelemen

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

• Ductile shear zones are defined by regions of localized strain.

• Play a critical role in the dynamics of the lithosphere and asthenosphere

• Exist over a wide range of scales, identified primarily on the basis of field relations and microstructure

Vauchez et al. (2012)

Webber et al. (2010)

~104 m

~10-2 m

Vauchez et al. (2012)

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

1. How do high temperature mantle shear zones form?

2. What are the microphysical mechanisms of weakening?

3. How does the strength of shear zones evolve with progressive deformation?

Skemer et al. (2010) JPet

g > 20g < 1

1 meter

2 mm

2 mm

ol

opx

ol

ol + opx

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

lowstrain

highstrain

1. How do high temperature mantle shear zones form?

2. What are the microphysical mechanisms of weakening?

3. How does the strength of shear zones evolve with progressive deformation?

Josephine Peridotite

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

“undeformed” harzburgite

strain markers

1 meter

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

strain gradientlow strain

high strain

Skemer et al. (2013) EPSL

PSZ gmax = 5, width ~40 m

GSZ gmax > 20, width ~15 m

ASZ gmax > 20, width ~5 m

Microstructural data for PSZ from Warren et al. (2008),GSZ from Skemer et al. (2010), and ASZ from Recanati et al (2012)

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Skemer et al. (2013) EPSL

“dry” “wet”

Plausible weakening mechanisms:

1. Grain size reduction

2. Shear heating

3. Partial melt

4. Water

5. Viscous anisotropy (LPO)

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

strain dependent viscous anisotropy

grain size

water concentration

melt fraction

temperature and pressure

Plausible weakening mechanisms:

1. Grain size reduction

2. Shear heating

3. Partial melt

4. Water

5. Viscous anisotropy (LPO)

g = 0.65

g = 5.25

Warren et al. (2008) EPSL

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Plausible weakening mechanisms:

1. Grain size reduction

2. Shear heating

3. Partial melt

4. Water

5. Viscous anisotropy (LPO)

tabular dunite (relict of channelized melt)

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

PSZ

Hirth and Kohlstedt (2003)

2.1

OHCPlausible weakening mechanisms:

1. Grain size reduction

2. Shear heating

3. Partial melt

4. Water

5. Viscous anisotropy (LPO)

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Ion probe measurements of water concentration show:

1) Gradients across individual shear zones (10s of meters)

2) Variation between individual shear zones (100s of meters)

3) Correlated with olivine LPO

Skemer et al. (2013) EPSL

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Plausible weakening mechanisms:

1. Grain size reduction

2. Shear heating

3. Partial melt

4. Water

5. Viscous anisotropy (LPO)

Tommasi et al. (2009)

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Durham and Goetze (1977)

Hansen et al. (2012) Nature

The magnitude of viscous anisotropy is proportional to the strength of the deformation induced LPO

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

1D Model:

1) Water introduced as zone of constant concentration. Diffusion causes shear zone to broaden.

2) Water content, shear stress (7 MPa), temperature (1000 C), viscous anisotropy incorporated into flow law:

3) At each time step water concentration and strain profiles calculated.

4) Results compared to PSZ (broadest shear zone analyzed)

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

time

Shear zone initial water concentration: Ci = 350 ppm H/SiFar field water concentration: Cb = 270 ppm H/Si

Effect of water alone Effect of water + viscous anisotropy

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Skemer et al. (2013) EPSLtime

Skemer et al. (2013) EPSL

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Effect of time-dependent water concentration + viscous anisotropy

GSZ

Large perturbations in viscosity and strain can be generated by gradients in water concentration.

These perturbations can be amplified by other strain-weakening effects:

o Viscous anisotropyo Shear heatingo Grain size reduction

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

PSZ

How do rocks evolve from coarse-grained monomineralic domains to fine-grained intermixed polymineralic domains?

What is required to generate long-lived, weak shear zones through the mantle lithosphere?

?

2 mm2 mm

ol

opx

ol

ol + opx

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Lanzo Massif

J. Linckens

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Linckens, Bruijn, Skemer (2014) EPSL

Initial Microstructure Final Microstructure

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Linckens, Bruijn, Skemer (2014) EPSL

Linckens, Bruijn, Skemer (EPSL, in press)

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Olivine

Linckens, Bruijn, Skemer (EPSL, in press)

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Orthopyroxene

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Linckens, Bruijn, & Skemer (EPSL)

olivine

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation

Linckens, Bruijn, Skemer (2014) EPSL

opx

Skemer et al. (2010) JPet

1. Strain perturbation generated by compositional heterogeneity

2. Amplified by viscous anisotropy

3. Preserved by grain-size reduction and phase mixing.

A Conceptual Model of Shear Zone Evolutionlow

strain

high strain

Introduction Shear Zone Preservation ConclusionsShear Zone Initiation