Strain localisation in the subcontinental mantle — a ductile alternative to the brittle mantle

Précigout, Jacques; Gueydan, Frédéric; Gapais, Denis; Garrido, Carlos J.; Essaifi, Abderrahim

doi:10.1016/j.tecto.2007.09.002

Cited by 110 publications

(118 citation statements)

References 77 publications

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“…Ultimately, strain weakening associated with localization may result from (e.g., Burlini and Bruhn, 2005): (1) grain size reduction by dynamic recrystallization or metamorphic reaction that produce new stress-free grains and/or a switch to grain size-sensitive deformation mechanisms, (2) structural softening in polyphase materials induced by the formation of interconnected weak layers at high strain, (3) geometric softening resulting from shape and crystallographic preferred orientation, (4) chemical weakening resulting in a change of point defects concentration, (5) fluid-induced dissolution-precipitation creep or reduction of the effective pressure causing embrittlement, (6) partial melting, leading to mechanical weakening and fast diffusion along wet grain boundaries, (7) transformation plasticity caused by mineral phase change, and (8) shear heating. In nature, the formation of a shear zone may be associated with a combination of these mechanisms, which holds also for the evolution of shear zones in the uppermost mantle (Kruckenberg et al, 2013;Newman and Drury, 2010;Precigout et al, 2007;Skemer et al, 2010).…”

Section: Introductionmentioning

confidence: 99%

Strain localization during high temperature creep of marble: The effect of inclusions

et al. 2014

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The deformation of rocks in the Earth's middle and lower crust is often localized in ductile shear zones. To better understand the initiation and propagation of high-temperature shear zones induced by the presence of structural and material heterogeneities, we performed deformation experiments in the dislocation creep regime on Carrara marble samples containing weak (limestone) or strong (novaculite) second phase inclusions. The samples were mostly deformed in torsion at a bulk shear strain rate of ≈ 1.9x10 -4 s -1 to bulk shear strains γ between 0.02 and 2.9 using a Paterson-type gas deformation apparatus at 900°C temperature and 400 MPa confining pressure. At low strain, twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at γ > 0.1 -0.2.Peak shear stress at the imposed conditions is about 20 MPa, which is ≈8% lower than the strength of intact samples. Strain progressively localized within the matrix with increasing bulk strain, but decayed rapidly with increasing distance from the inclusion tip.Microstructural analysis shows twinning and recrystallization within this process zone, with a strong crystallographic preferred orientation, dominated by {r} and (c) slip in .Recrystallization-induced weakening starts at local shear strain of about 1 in the process zone, corresponding to a bulk shear strain of about 0.1. In contrast, torsion of a sample containing strong inclusions deformed at similar stress as inclusion-free samples but do not show localization. The experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips. Recrystallizationinduced grain size reduction may only locally promote grain boundary diffusion creep. Accordingly, the bulk strength of the twisted aggregate is close to or slightly below the lower (isostress) strength bound, determined from the flow strength and volume fraction of matrix and inclusions.

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Section: Introductionmentioning

confidence: 99%

Strain localization during high temperature creep of marble: The effect of inclusions

et al. 2014

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“…The interplay between deformation, grain size decrease, and mechanical weakening is considered as a major process to account for the localization of the ductile deformation of rocks in the shear zones (e.g., Boullier and Gueguen 1975;Rice 1976;Poirier 1980;Braun et al 1999;Furusho and Kanagawa 1999;Precigout et al 2007;Raimbourg et al 2008). Though the operating microscopic mechanisms are not constant but depend on strain rate and temperature (Hirth and Tullis 1992;Poirier 1995), grain size reduction of constituting minerals involves the creation of a large area of grain boundaries (GBs) and is therefore ultimately controlled by GB structure and properties.…”

Section: Introductionmentioning

confidence: 99%

Subgrain boundary analyses in deformed orthopyroxene by TEM/STEM with EBSD-FIB sample preparation technique

et al. 2014

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High-resolution structure analyses using electron beam techniques have been performed for the investigation of subgrain boundaries (SGBs) in deformed orthopyroxene (Opx) in mylonite from Hidaka Metamorphic Belt, Hokkaido, Japan, to understand ductile deformation mechanism of silicate minerals in shear zones. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analysis of Opx porphyroclasts in the mylonitic rock indicated that the crystal orientation inside the Opx crystals gradually changes by rotation about the b-axis by SGBs and crystal folding. In order to observe the SGBs along the b-axis by transmission electron microscopy (TEM) or scanning TEM (STEM), the following sample preparation protocol was adopted. First, petrographic thin sections were slightly etched with hydrofluoric acid to identify SGBs in SEM. The Opx crystals whose b-axes were oriented close to the normal of the surface were identified by EBSD, and the areas containing SGBs were picked and thinned for (S) TEM analysis with a focused ion beam instrument with micro-sampling system. High-resolution TEM imaging of the SGBs in Opx revealed various boundary structures from a periodic array of dissociated (100) [001] edge dislocations to partially or completely incoherent crystals, depending on the misorientation angle. Atomic-resolution STEM imaging clearly confirmed the formation of clinopyroxene (Cpx) structure between the dissociated partial dislocations. Moreover, X-ray microanalysis in STEM revealed that the Cpx contains a considerable amount of calcium replacing iron. Such chemical inhomogeneity may limit glide motion of the dislocation and eventually the plastic deformation of the Opx porphyroclasts at a low temperature. Chemical profiles across the high-angle incoherent SGB also showed an enrichment of the latter in calcium at the boundary, suggesting that SGBs are an efficient diffusion pathway of calcium out of host Opx grain during cooling.

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“…Recent observations on natural samples highlighted the importance of the grain-size-dependent dislocation-accommodated grain boundary sliding (disGBS) of olivine as controlling the rheology of mantle shear zone (Précigout et al, 2007;Warren and Hirth, 2006). Deformation experiments and numerical investigations have also shown that disGBS could promote strain localisation during dynamic recrystallization (Hirth and Kohlstedt, 2003;Précigout and Gueydan, 2009).…”

Section: / Geological Constraintsmentioning

confidence: 99%

“…Deformation experiments and numerical investigations have also shown that disGBS could promote strain localisation during dynamic recrystallization (Hirth and Kohlstedt, 2003;Précigout and Gueydan, 2009). Indeed, under conditions where disGBS constitutes the dominant deformation mechanism of peridotite, i.e., at temperature lower than 800°C, grain size reduction is associated with a significant drop of strength ( (Précigout et al, 2007), see Appendix for more details). This amount of strain weakening increases with decreasing temperature and does not occur for temperature larger than 800°C.…”

Section: / Geological Constraintsmentioning

confidence: 99%

“…Based on field observations in the Ronda peridotite (southern Spain) and the experimental data of Hirth and Kohlstedt (2003), Précigout et al (2007) proposed a strain-dependent mantle rheology in which the overall strain rate ̇ is the sum of four ductile deformation mechanisms: dislocation creep ( ̇), diffusion creep ( ̇), disGBS ( ̇) and exponential creep ( ̇) (Hirth and Kohlstedt, 2003;Drury, 2005;Précigout et al, 2007). Each mechanism contributes to the bulk strain rate ( ) of an olivine aggregate according to:…”

Section: / Mantle Rheologymentioning

confidence: 99%

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Strain weakening enables continental plate tectonics

2014

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Much debate exists concerning the strength distribution of the continental lithosphere, how it controls lithosphere-scale strain localization and hence enables plate tectonics. No rheological model proposed to date is comprehensive enough to describe both the weakness of plate boundary and rigid-like behaviour of plate interiors. Here we show that the duality of strength of the lithosphere corresponds to different stages of microstructural evolution. Geological constraints on lithospheric strength and large strain numerical experiments reveal that the development of layers containing weak minerals and the onset of grain boundary sliding upon grain size reduction in olivine cause strain localisation and reduce strength in the crust and subcontinental mantle, respectively. The positive feedback between weakening and strain localization leads to the progressive development of weak plate boundaries while plate interiors remain strong. 2 1/ IntroductionThe extrapolation of laboratory flow laws to geological scale suggests a complex layering of brittle and ductile layers within the continental lithosphere (Brace and Kohlstedt, 1980;Sawyer, 1985). For classical continental geotherm, the upper lithospheric mantle is expected to be brittle and support high stresses. Many analogue and numerical experiments indicate that such a rheological layering is important to reproduce first-order patterns of lithosphere deformation (Brun, 1999;Burov and Yamato, 2008). In particular, the presence of a brittle uppermost mantle is needed to explain strain localisation at lithospheric scale (Buck, 1991;Gueydan et al., 2008).However, recent geophysical studies question this classical view of the continental strength layering. Based on earthquakes distribution and elastic thicknesses of the continental lithosphere, including cratons, it has been proposed that the uppermost mantle could behave as ductile instead of brittle (Déverchère et al., 2001;Jackson, 2002;Maggi et al., 2000). However, the mechanical stability of cratons requires that the uppermost mantle supports high stresses (Burov, 2010). In addition, the post-seismic displacement field, i.e., the pattern of deformation at the surface of the Earth within weeks to years following an earthquake, suggests that the deep crust is stronger than the lithospheric mantle. (Bürgmann and Dresen, 2008;Thatcher and Pollitz, 2008). Note however that post seismic displacement field may also result from a complex combination of poro-elasticity and fault creep in the seismogenic layer and viscous flow in the lower crust (Barbot and Fialko, 2010). In this case, it may not be used to constrain strength ratios between the ductile crust 3 and lithospheric mantle. Finally, the recent discovery of non-volcanic tremors, i.e., long duration seismic events with small amplitudes, below the San Andreas fault suggests a zone of localized and easily modulated faulting in the lower crust (Nadeau and Guilhem, 2009;Thomas et al., 2009). Assuming high temperatures in plate boundaries such as the San Andreas...

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Strain localisation in the subcontinental mantle — a ductile alternative to the brittle mantle

Cited by 110 publications

References 77 publications

Strain localization during high temperature creep of marble: The effect of inclusions

Strain localization during high temperature creep of marble: The effect of inclusions

Subgrain boundary analyses in deformed orthopyroxene by TEM/STEM with EBSD-FIB sample preparation technique

Strain weakening enables continental plate tectonics

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