A better comprehension of the rheology of the lithosphere is required to relate long and short term deformation regimes and describe the succession of events that lead to earthquake generation. But our vision of the rheology is blurred because gaps exist between the visions of geologists, experimentalists and modellers. It seems now clear that there is not one rheological model applicable to all contexts and that rheological parameters should be adapted to each situation. We plan to study the rheological behaviour of real objects taking into account their particular lithological, kinematic and thermodynamic characteristics. RHEOLITH is designed to assess the rheology of natural structures from field observation to lab experiments and numerical modelling, focussing on two situations where rheological changes take place, accessible to our experimental capabilities, brittle-ductile transition and anatexis. We will work on exhumed crustal-scale shear zones and describe them in their natural complexity, focussing our attention on strain localisation and high strain structures that can eventually lead to fast slip events. A number of objects will be studied, starting from geological description (3D geometry, P-T-fluids estimates and dating), experimental studies of rheological properties of natural sampled rocks and numerical modelling. We will set an Argon-dating lab to work on very dense sampling for dating along strain gradients in order to overcome local artefacts, to quantify strain rates and rates of strain localisation. We will then deform in the lab natural rocks taken from the studied objects as well as synthetic samples in order to retrieve the rheological parameters adapted to those objects. We will finally numerically model processes at various scales, from the lab to the lithosphere in order to ensure a clean transfer of rheological parameters from one scale to another. Our approach is original and the past achievements of the PI and the CoPI in studying geodynamic topics both in the field and through numerical modelling and the association in Orléans of field geologists with a internationally recognized team working on experimental petrology and rock mechanics give RHEOLITH the best chances to succeed. The main impact of RHEOLITH will be the quantification of rheological parameters and the description of localisation processes adapted to real geological situations. This project requires to update the equipment available at Institut de Sciences de la Terre d’Orléans (PI : Laurent JOLIVET ; ISTO, Université d’Orléans) and Institut des Sciences de la Terre de Paris (co-PI : Evguenii BUROV, ISTEP, UPMC-Paris 6). To complement experimental facilities at ISTO a second Paterson rig will be acquired in order to allow the association of short-term (a few days to a few weeks) and long-term (a few weeks to a few months) experiments. In order to obtain numerous argon ages one argon-line will be acquired at ISTO, for in situ laser dating and for the step-heating approach to complement the existing two spectrometers. Finally, a faster computing system is needed at ISTEP to acquire the numerical capabilities to work on different space and time scales.