The Role of Pyroxenites in Formation of Shear Instabilities in the Mantle: Evidence from an Ultramafic Ultramylonite, Twin Sisters Massif, Washington

Toy, Virginia; Newman, Joseph W.; Lamb, William M.; Tikoff, Basil

doi:10.1093/petrology/egp059

Cited by 55 publications

(61 citation statements)

References 81 publications

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“…4A). Furthermore, the observed efficient mixing of recrystallized olivine and orthopyroxene grains in our study is consistent with the microstructures observed in sheared peridotites (2,3), suggesting that our observations and model offer a reasonable means of assessing the potential for shear weakening across a range of temperature-stress-strain rate conditions. However, natural observations are inconsistent with a model prediction based on pure olivine grain-growth kinetics (29) (Fig.…”

Section: Significancesupporting

confidence: 86%

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Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization

Farla

Karato

Cai

2013

Proc. Natl. Acad. Sci. U.S.A.

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For plate tectonics to operate on a terrestrial planet, the surface layer (the lithosphere) must have a modest strength (Earth, ≤200 MPa), but a standard strength profile based on olivine far exceeds this threshold value. Consequently, it is essential to identify mechanisms that reduce the strength of the lithosphere on Earth. Here we report results of high-strain laboratory deformation experiments on a representative olivine-orthopyroxene composition that show the addition of orthopyroxene substantially reduces the strength in the ductile regime within a certain temperature window. The reduction in strength is associated with the formation of small orthopyroxene and olivine grains. Our samples show heterogeneous microstructures similar to those observed in natural peridotites in shear zones: fine-grained regions containing both orthopyroxene and olivine that form interconnected bands where a large fraction of strain is accommodated. A model is developed to apply these results to geological conditions. Such a model, combined with our experimental observations, suggests that orthopyroxene may play a key role in the plastic deformation of the lithosphere in a critical temperature range, leading to long-term weakening associated with strain localization in the lithosphere.rheology | two-phase | Zener pinning | deformation mechanisms | electron probe microanalysis P late tectonic style of convection is characterized by regions of localized deformation, such as subduction zones, and the origin of these regions needs to be investigated to understand why plate tectonics operates on Earth. Localized deformation at plate boundaries involves not only brittle fracture at shallow portions but also localized deformation in the ductile shear zones (1-4). Although the basic processes of localized deformation in the brittle regime are well understood (5, 6), mechanisms of localized deformation in the ductile regime remain elusive. The strength profile of the lithosphere in a standard model (7) predicts stresses that are much higher than the critical strength below which plate tectonic style of convection would occur (8,9).Among the possible mechanisms of shear localization (10), grain size reduction is most often recognized in ductile shear zones and has been given particular attention in previous studies (11)(12)(13)(14)(15). Grain size reduction can occur during high-temperature deformation by dynamic recrystallization during dislocation creep (16,17). In many cases, small grains are formed along preexisting grain boundaries (18). If the degree of grain-size reduction is large enough and these fine-grained regions are connected, then regions of small grain size will be deformed by grain-size-sensitive creep processes-diffusion creep or dislocation creep accommodated by grain-boundary sliding (Dis-GBS) introduced by Hirth and Kohlstedt (1995) (19). Deformation in both regimes results in the weakening of a rock, although less so in the Dis-GBS regime. In addition, for this weakening to cause substantial shear localization, the ...

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Section: Significancesupporting

confidence: 86%

“…Localized deformation at plate boundaries involves not only brittle fracture at shallow portions but also localized deformation in the ductile shear zones (1)(2)(3)(4). Although the basic processes of localized deformation in the brittle regime are well understood (5,6), mechanisms of localized deformation in the ductile regime remain elusive.…”

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confidence: 99%

Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization

Farla

Karato

Cai

2013

Proc. Natl. Acad. Sci. U.S.A.

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“…It has been shown in experimentally (Farla et al, 2013;Hiraga et al, 2010a;Ji et al, 2001;Mcdonnell et al, 2000;Sundberg and Cooper, 2008;Tasaka et al, 2013) and naturally deformed upper mantle rocks (Linckens et al, 2011b;Skemer et al, 2010;Tasaka et al, 2014;Toy et al, 2010;Warren and Hirth, 2006), as well as in theoretical models (Bercovici and Ricard, 2012) that the presence of secondary minerals may lead to substantially different rheological behavior.…”

Section: Introductionmentioning

confidence: 99%

Small quantity but large effect — How minor phases control strain localization in upper mantle shear zones

2015

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“…Dynamic recrystallization of study peridotite occurred very likely at elevated stress conditions leading to significant grain size reduction down to 50 m in diameter from an original coarse-grained microstructure in the intra-mantllic shear zone. Microstructural analyses of other peridotite mylonites indicate that shear localization results from the combined effects of grain size reduction, grain boundary sliding and second phase pinning during deformation (Kennedy et al, 2002;Toy et al, 2010;Warren and Hirth, 2006). Microstructural analyses of the mylonite fabric in study as well as other peridotite bodies suggest that deformation of aggregates with a grain size of 10-100 m occurred by dislocation-accommodated grain boundary sliding which produces an LPO.…”

Section: Rheological Constraints For Mechanical Behavior Of Peridotitmentioning

confidence: 87%

Ductile deformation and rheology of sub-continental mantle in a hot collisional orogeny: Example from the Bohemian Massif

Kusbach¹,

Ulrich²,

Schulmann³

2012

Journal of Geodynamics

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The Role of Pyroxenites in Formation of Shear Instabilities in the Mantle: Evidence from an Ultramafic Ultramylonite, Twin Sisters Massif, Washington

Cited by 55 publications

References 81 publications

Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization

Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization

Small quantity but large effect — How minor phases control strain localization in upper mantle shear zones

Ductile deformation and rheology of sub-continental mantle in a hot collisional orogeny: Example from the Bohemian Massif

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