Post on 16-Jan-2016
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