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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  05 Jun 2020

05 Jun 2020

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A revised version of this preprint is currently under review for the journal SE.

Relationship between microstructures and resistance in mafic assemblages that deform and transform

Nicolas Mansard1, Holger Stünitz1,2, Hugues Raimbourg1, Jacques Précigout1, Alexis Plunder3, and Lucille Nègre1 Nicolas Mansard et al.
  • 1Institut des Sciences de la Terre d'Orléans (ISTO), UMR 7327, CNRS/BRGM, Université d'Orléans, 45071 Orléans, France
  • 2Department of Geology, University of Tromsø, Dramsveien 201, 9037 Tromsø, Norway
  • 3BRGM, F-45060, Orléans, France

Abstract. Syn-kinematic mineral reactions play an important role for the mechanical properties of polymineralic rocks. Mineral reactions (i.e. nucleation of new phases) may lead to grain size reduction producing fine-grained polymineralic mixtures, which have a strongly reduced viscosity because of the activation of grain-size sensitive deformation processes. In order to study the effect of deformation-reaction feedback(s) on sample strength, we performed rock deformation experiments on wet assemblages of mafic compositions in a Griggs-type solid-medium deformation apparatus. Shear strain was applied at constant strain rate (10−5 s−1) and constant confining pressure (1 GPa) with temperatures ranging from 800 to 900 °C. At low shear strain, the assemblages that react faster are significantly weaker than the ones that react more slowly, demonstrating that reaction progress has a first-order control on rock strength. With increasing strain, we document two contrasting microstructural scenarios: (1) the development of a single through-going high-strain-zone of well-mixed, fine-grained aggregates, associated with a significant weakening after peak stress and (2) the development of partially connected, nearly monomineralic shear bands without major weakening. The lack of weakening is caused by the absence of interconnected well-mixed aggregates of fine-grained reaction products. The nature of the reaction products, and hence the intensity of the mechanical weakening, is controlled by the microstructures of the reaction products to a large extent, e.g., the amount of amphibole and the phase distribution of reaction products. The samples with the largest amount of amphibole exhibit a larger grain size and show less weakening. In addition to their implications for the deformation of natural shear zones, our findings demonstrate that the feedback between deformation and mineral reactions can lead to large differences in mechanical strength, even at relatively small initial differences in mineral composition.

Nicolas Mansard et al.

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Nicolas Mansard et al.

Nicolas Mansard et al.


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Latest update: 20 Sep 2020
Publications Copernicus
Short summary
Our rock deformation experiments (Solid-medium Griggs type apparatus) on wet assemblages of mafic compositions show that the ability of minerals to react controls the portions of rocks that deform, and that minor chemical and mineralogical variations can considerably modify the strength of deformed assemblages. Our study also suggests that the rheology of mafic rocks which constitutes a large part of the oceanic crust cannot be summarized as being rheologically controlled by monophase materials.
Our rock deformation experiments (Solid-medium Griggs type apparatus) on wet assemblages of...