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Initiation and preservation of localized deformation in the mantle
Phil Skemer Washington University in St. Louis Structural Geology and Tectonics Forum June 16, 2014 With contributions from: Rolf Bruijn, Jolien Linckens, Jessica Warren, Lars Hansen, Greg Hirth, Peter Kelemen
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Ductile shear zones are defined by regions of localized strain.
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Vauchez et al. (2012) Webber et al. (2010) ~104 m ~10-2 m 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)
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1. How do high temperature mantle shear zones form?
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions 1 meter 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? g < 1 g > 20 Skemer et al. (2010) JPet
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1. How do high temperature mantle shear zones form?
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions ol 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? low strain opx 2 mm ol high strain ol + opx 2 mm
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Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Josephine Peridotite
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“undeformed” harzburgite
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions “undeformed” harzburgite low strain strain markers strain gradient high strain 1 meter
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PSZ gmax = 5, width ~40 m GSZ gmax > 20, width ~15 m
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions 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) Skemer et al. (2013) EPSL
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Shear Zone Preservation Conclusions
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions “dry” “wet” Skemer et al. (2013) EPSL
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temperature and pressure
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Plausible weakening mechanisms: Grain size reduction Shear heating Partial melt Water Viscous anisotropy (LPO) grain size melt fraction strain dependent viscous anisotropy water concentration temperature and pressure
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Plausible weakening mechanisms:
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Plausible weakening mechanisms: Grain size reduction Shear heating Partial melt Water Viscous anisotropy (LPO) g = 0.65 g = 5.25 Warren et al. (2008) EPSL
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Plausible weakening mechanisms:
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions PSZ Plausible weakening mechanisms: Grain size reduction Shear heating Partial melt Water Viscous anisotropy (LPO) tabular dunite (relict of channelized melt)
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Plausible weakening mechanisms:
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Plausible weakening mechanisms: Grain size reduction Shear heating Partial melt Water Viscous anisotropy (LPO) Hirth and Kohlstedt (2003)
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Ion probe measurements of water concentration show:
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Ion probe measurements of water concentration show: Gradients across individual shear zones (10s of meters) Variation between individual shear zones (100s of meters) Correlated with olivine LPO Skemer et al. (2013) EPSL
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Plausible weakening mechanisms:
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Plausible weakening mechanisms: Grain size reduction Shear heating Partial melt Water Viscous anisotropy (LPO) Durham and Goetze (1977) Tommasi et al. (2009)
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Introduction Shear Zone Initiation Shear Zone Preservation Conclusions The magnitude of viscous anisotropy is proportional to the strength of the deformation induced LPO Hansen et al. (2012) Nature
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1D Model: Introduction Shear Zone Initiation Shear Zone Preservation
Conclusions 1D Model: Water introduced as zone of constant concentration. Diffusion causes shear zone to broaden. Water content, shear stress (7 MPa), temperature (1000 C), viscous anisotropy incorporated into flow law: At each time step water concentration and strain profiles calculated. Results compared to PSZ (broadest shear zone analyzed) time
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Shear zone initial water concentration: Ci = 350 ppm H/Si
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Shear zone initial water concentration: Ci = 350 ppm H/Si Far field water concentration: Cb = 270 ppm H/Si Effect of water alone Effect of water + viscous anisotropy time Skemer et al. (2013) EPSL
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Effect of time-dependent water concentration + viscous anisotropy
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Effect of time-dependent water concentration + viscous anisotropy Skemer et al. (2013) EPSL
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Introduction Shear Zone Initiation Shear Zone Preservation Conclusions PSZ Large perturbations in viscosity and strain can be generated by gradients in water concentration. These perturbations can be amplified by other strain-weakening effects: Viscous anisotropy Shear heating Grain size reduction GSZ
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Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Lanzo Massif ol ol ol + opx ? opx 2 mm 2 mm J. Linckens 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?
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Shear Zone Preservation Conclusions
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Linckens, Bruijn, Skemer (2014) EPSL
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Shear Zone Preservation Conclusions
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Initial Microstructure Final Microstructure Linckens, Bruijn, Skemer (2014) EPSL
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Olivine Introduction Shear Zone Initiation Shear Zone Preservation
Conclusions Olivine Linckens, Bruijn, Skemer (EPSL, in press)
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Orthopyroxene Introduction Shear Zone Initiation
Shear Zone Preservation Conclusions Orthopyroxene Linckens, Bruijn, Skemer (EPSL, in press)
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Introduction Shear Zone Initiation Shear Zone Preservation Conclusions Linckens, Bruijn, & Skemer (EPSL)
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olivine opx Introduction Shear Zone Initiation Shear Zone Preservation
Conclusions olivine opx Linckens, Bruijn, Skemer (2014) EPSL
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A Conceptual Model of Shear Zone Evolution
Introduction Shear Zone Initiation Shear Zone Preservation Conclusions A Conceptual Model of Shear Zone Evolution low strain Strain perturbation generated by compositional heterogeneity Amplified by viscous anisotropy Preserved by grain-size reduction and phase mixing. high strain Skemer et al. (2010) JPet
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