The Institut des Sciences de la Terre d’Orléans invites for application for an ERC funded three-years PhD thesis in Geodynamics – Radiometric Dating

The Institut des Sciences de la Terre d’Orléans invites for application for an ERC funded three-years PhD thesis in Geodynamics – Radiometric Dating

The brittle-ductile transition in detachments and strike-slip shear zones: evolution of rheological behaviour during strain localisation

Key words: rheology, brittle-ductile-transition, detachments, strike-slip, shear zone, 40Ar-39Ar, strain localisation, radiometric dating, laboratory experiments

Project description: In classical rheological models, the brittle-ductile transition (BDT) corresponds to a peak of resistance at the intersection of rupture criteria characteristic of the upper crust and flow laws at work in the lower crust. However, field observations shows that large-scale detachments often tend to localise precisely in the BDT in the P-T conditions of the lower greenschists facies, where they accommodate large displacements before entering the brittle field during exhumation. Previous studies have shown the importance of various
factors for strain localisation such as grain size changes, fluid-rock interactions and tectonic inheritance. Fluidrock interactions play a major role because the presence of fluids may modify the effective pressure, because the incorporation of water in crystal lattice decreases the resistance and also because fluids control metamorphic reactions that produce weak mineral phases such as phyllosilicates. When modelling the formation of large-scale detachments, one often has to use ad-hoc rheological parameters to empirically describe how rock materials weaken at the depth of the brittle-ductile transition. The various processes involved in strain localisation can be observed on exhumed rocks that have crossed the ductile-to brittle transition during exhumation as in the case of detachments (Cyclades, Tuscan Archipelago, Basin and Range, Norwegian Caledonides) but also in the case of strike-slip shear zones such as the South Armorican Shear Zone. The comparison of both contexts is interesting because exhumation rates and the interactions with surface-derived fluids are very different, thus providing the opportunity to study and quantify their rheological consequences independently. In the case of strike-slip shear zone one can even see alternating ductile and brittle behaviour depending upon strain rate. The aim of the research is to quantify the role of such localising factors based on field observations of extensional structures and a dense and detailed sampling across shear zones to estimate the P-T conditions of syn-kinematic parageneses and the timing of strain localisation. The study will combine detailed field analysis of a limited number of shear zones, metamorphic petrology, age-constraints (40Ar-39Ar and Rb-Sr methods) and laboratory experiments.

The host institution and the laboratory: The University of Orléans (UO) is a medium-size campus with 13000 students in different fields of research from Mathematics, Physics, Law, Economy, Biology to Earth and Space Sciences. The laboratory (Institut des Sciences de la Terre d’Orléans, ISTO) is part of the Observatoire des Sciences de l’Univers en Région centre (OSUC), an internal school of the UO devoted to research and teaching in geosciences, environmental sciences, atmospheric and space sciences. ISTO has developed research facilities in geodynamics and experimental petrology that are internationally recognized as proven by one ERC Advanced Research Grant RHEOLITH and two ERC Starting Research Grants. The OSUC has also been recently elected as a centre of excellence via two nationally-funded projects (Labex VOLTAIRE dedicated to the study of fluids from the deep Earth to the atmosphere, and Equipex PLANEX dedicated to analyses and experiments in extreme environments) showing the steadily growing role played by ISTO at the national level.
RHEOLITH will involve ~6 academic staffs, 7 PhD students, 3 engineers and 3 post-docs thus providing a strongly integrated and interactive academic-research environment for the PhD project .

Eligibility: We are seeking for a creative individual with a diploma or M.Sc. degree in geology. A solid background in geodynamics and petrology is an advantage.

Funding context: the research and the scholarship will be funded by the RHEOLITH ERC Advanced Research Grant (P.I. L. Jolivet, co-PI E. Burov)

Application: Requests and applications including a motivation letter, CV, as well as the names and email addresses of two potential referees should be submitted as a single pdf file by email to Laurent Jolivet (Laurent.jolivet@univ-orleans.fr), Romain Augier (romain.augier@univ-orleans.fr), Stéphane Scaillet (stephane.scaille@cnrs-orleans.fr). Deadline for application is June 30, 2013.

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