Missions

Earth surface dynamic is governed by the relative displacement of tectonic plates. Deformation is mainly localized at plate boundaries, in zones of strain localization called shear zones, mechanically weaker than plate interiors -although composed of the same material- and able to accommodate tens to hundreds of kilometers of relative displacement. In the shear zones of the middle and upper continental crust, micas are systematically observed in zones of strain localization (Tullis and Wenk, 1994; Hunter et al., 2016 ; Graziani et al., 2020 ; Alaoui et al., 2023). The (re)crystallization of mica whose layered structure makes it mechanically weaker than quartz and feldspar (Bons, 1988 ; Kronenberg et al., 1990) has therefore been described as a rock weakening reaction. However, little is known about the actual mechanical strength of mica and mica-bearing assemblages. Micas in mylonites often have a strong crystallographic fabric suggesting plastic deformation by sliding along the cleavage plane (001), but direct pieces of evidence, such as dislocations, are most often lacking. In addition, other processes, such as oriented growth, can account for the crystallographic fabric (Stallard and Shelley, 2005). Micas in mylonites can also show strong grain size reduction, but the underlying processes are not understood. This lack of knowledge is mainly due to a lack of deformation experiments on micas, which mainly focused so far on the low-temperature, friction realm (Moore et al., 1989 ; den Hartog et al., 2010), whereas high-pressure and high-temperature laboratory experiments, relevant to the viscous deformation prevalent in the middle crust, are very limited (Holyoke and Tullis, 2006 ; Shea and Kronenberg, 1993 ; Tokle et al., 2022).

The objective of this postdoctoral project is to investigate the mechanisms of deformation of mica in middle crustal conditions, with a special focus on the influence of dissolution-precipitation and grain boundary processes. These elements will then be integrated into a model of polymineralic rock aggregate, in order to quantify the rheological effect of the presence of mica on polyphased rock strength. The work will merge different approaches and techniques, including the experimental deformation of mica-bearing aggregates in the Griggs apparatus and micro to nano-scale analyses of compositional variations, density and type of defects in mica.

Activities

• Perform simple shear experiment in Griggs apparatus on quartz+mica aggregates. Variable proportions and variable composition of micas will be explored.
• Analyze and interpret the microstructures of deformation in micas using a large set of techniques, including SEM, EBSD and TEM.
• Describe quantitatively the geometrical arrangement of grains and connect it to the processes of grain size reduction and deformation;
• Analyze the reactional/chemical processes occurring at mica grain boundaries, using EPMA, STEM and nano-SIMS;
• Describe in a quantitative way the rheology of the deformed mica assemblages.

Skills

• Phd in Earth Science, material science or solid mechanics
• Experience with at least one micro or nano-analytical technique among SEM, EBSD, Microprobe, TEM, Raman etc.
• Written and oral communication skills in English.

Work Context

This position is funded by the ANR project WeakStRock (ANR-23-CE49-0005) which aims to unravel the processes of mechanical weakening in shear zones of the continental crust and in particular the role of phyllosilicates in the deformation of polyphase crustal rocks. One of the main objectives of this project is to identify and quantify the effect of the chemical interactions among solid phases and solids and fluids at grain boundaries on crustal rock rheology and strain accommodation mechanisms. The postdoc project will work in close collaboration with a PhD student (already recruited in the framework of WeakStRock), whose work focuses principally on the deformation process of quartz in mica+quartz aggregates.

The work will be carried out at the Institut of Science de la Terre d'Orléans (ISTO). The recruited postdoctoral researcher will work in the MadMAx (Magma and deformation) team, in close collaboration with 3-4 researcher of the permanent staff (associate professors, professors and engineers). ISTO also benefits from a large range of experimental equipment including three Griggs apparatuses (among which one new generation Griggs), one Paterson apparatus, internally-heated pressure vessels, in-situ X-ray/Raman/Infra-Red devices. These experimental facilities are complemented by various analytical equipment such as: SEM, EBSD, EPMA with hyperspectral cathodoluminescence, optical cathodoluminescence, micro-Raman, TEM and FIB.

Additional Information

Applications needs to include a CV and a motivation letter reporting the recent research work of the candidate and his/her statements of interest and possible contributions for the project.