F Dubois

M Jean

M Renouf

R Mozul
email: remy.mozul@univ-montp2.fr

A Martin

M Bagneris
email: marine.bagneris@univ-montp2.fr

0e Colloque National en Calcul des Structures 9-13 Mai 2011, Presqu'île de Giens (Var) LMGC90

Keywords: contact, dynamics, multiple physics, DEM, FEM, OpenSource, Fortran, Python

This paper gives a short overview of LMGC90 platform in terms of architecture and main features.

Introduction

LMGC90 is an open platform dedicated to the modeling of large collections of interacting objects (2D/3D). It aims at modeling objects of any shape with various mechanical behavior and to take into account interaction laws as complex as necessary. Furthermore multi-physics couplings (thermal effects, fluids, etc) are progressively taken into account. LMGC90 is designed as a research software which offers to developers the possibility to add new physical models (behavior law, interaction law, etc), numerical models (finite element, natural element, etc), technical features (contact detection, visualization, parallelism, etc) and numerical strategies (time integration, numerical solver, etc). Furthermore end-users are able, through the supervisor, to interact with the internal database and to use/modify these informations during the simulation. As all research software, versatility of LMGC90 software has been obtained designing a dedicated architecture, however it needs to evolve more or less deeply with the new scientific requirements. In the present paper we refer to the architecture of the second version.

Architecture

As summarized on FIGURE 1, LMGC90 is divided into independent parts : -Core which is dedicated to the simulation itself. The core of LMGC90 software is written in Fortran9x, which language is widely known as computationally efficient. Core provides a Fortran9x public interface allowing to drive a simulation, access to the database, etc. -a supervising part, which rely on the Core public API. The common usage is possible through the ChiPy Python module which is build on a C API provided by a Fortran200x wrapping of the Core public API. An advanced-user can directly write a standalone program calling the Core public API or can build new functionality which can be added to the standard ChiPy module. A Sandbox mechanism is proposed to manage these developments (compiling, versioning, etc). -Bindings which embed full external libraries. A general API has been defined for sets of key features on the Core side and a wrapping to some existing libraries to fulfill the desired functionality has been done on the Bindings side. Existing bindings concern linear algebra (use of Lapack as ExternalLinea-rAlgebra), bulk behavior (Matlib as ExternalModels) and contact detection algorithms (ExternalDetection). Various other bindings are also available (FE code, etc). -User module which permits to define, through Fortran9x functions, some very specific possibilities as the orthotropic frame, the value of some fields at Gauss points, etc.

Core

Core relies on various Fortran9x modules. A module gathers type definitions and methods working on data of these types. Moreover modules also hold data of these defined types in order to limit side effects.

Usually the data and types are private, and a set of public functions are available to use the module. Thus a structure close to an object (in the Oriented Object Programming sense) is obtained but without polymorphism or inheritance.

The general architecture of the Core part is depicted FIGURE 2. Core provides functionalities to describe the model and to run a simulation with it. The model is built using the preprocessor part of LMGC90 [START_REF] Martin | Conception d'un outil adapté à la mise en données des systèmes discrets, 10éme Colloque National en Calcul des Structures[END_REF].

The input data coming through MeliMelo from the preprocessor are heterogeneous (objects may be different) but self content (the consistency checks are made within the Pre) and need to be split into a physical model (Modelization) and locus of interaction (Contactor). A numerical model is a discretization in space of a physical model. It will be used to compute the physical evolution of a body. The data pushed by MeliMelo depend on the kind of model : -for a rigid body : position of the center of inertia, inertia matrix, boundary conditions, material parameters. -for a deformable meshed body : position of the nodes, connectivity of each element and for each element its model and material and boundary conditions A contactor contains a geometrical model of a potential interaction locus which can be described using simple convex primitives (disk, sphere, polygon, polyhedron, etc), a cluster of these primitives or a general triangulated surface. An object may have several contactors.

During a simulation the Interaction Handler module manages the creation of interactions parsing the contactors list and using smart contact detection algorithms. An interaction contains some geometrical data (position, local frame, gap, maps, etc) and modeling data (reaction, internal variables, etc). An other module, Model Handler, manages the creation of systems of equations (SoE) integrating in time the models contained in Modelization. These two handler modules are driven by the simulation which provides integration rules depending on the chosen time integrator. The current state of a bulk model or of an interaction is stored in a generic database : the State module. Numerical methods are provided by the Solver module to solve all the systems of equations while fulfilling the constraints given by the interactions.

Even if this structure sounds very simple, it is not so easy to implement since the modelization, being rigid or deformable (small or large deformation) are treated rather differently before obtaining a generic system of equations. Thus the Modelization module is in fact a kind of abstraction module on any numerical representation of a physical model. The Contactor module may be seen in the same manner with regards to the various geometric shapes of contactors. Either Modelization or Contactor provides a generic interface to different modules which implement the specific functionality for a given data type. The same logic is used for every set of module also one can imagine easily adding its new solver, time integrator, contactor geometry and the corresponding detection features.

Since the inheritance of objects is now supported in the Fortran200x norm, one could expect we use such a mechanism. But unfortunately Fortran compilers have still partially integrated the new norms and from one compiler to another the integrated features are not the same. Thus it seems irrelevant to rely on too new features proposed by the Fortran norm.

Some advanced programming features

As a foreword, we would like to say that developing a large Fortran software while trying to make it readable for new developers and friendly for users is quite difficult (almost painful). One reason comes from the fact that Fortran lacks of several really convenient features or mechanisms available in other languages (templates, polymorphism, inheritance,etc). As we noticed before some of them are present in the Fortran200x norm but not yet fully implemented in the available compilers. Moreover the performance of such advanced programming features is questionable. Thus our policy is to stay as close as possible to the Fortran9x norm in the Core's CPU consuming parts and progressively integrate some well supported features of the new norm, like the C language binding in the interface, to give us some flexibility in programming.

However some necessary programming features are missing and here is described the way we mimic them using Fortran : -template :

One of the most envied features of the C++ language, is the Standard Template Library (STL), which provides vector, list, queue, etc, for any data type. When building dynamic data structures a basic convenient structure is the linked list. With standard Fortran9x such structures can be created through user defined types and pointers. The issue raises from the fact that the mechanism is type dependent and can not be deferred to a module. One understand that copying the same subroutines in each place where linked list are needed is error prone and tedious to maintain. So, to mimic a STL like behavior in Fortran one needs to trickily mix various possibilities. Key features are the overloading of the name of a type and the possibility to include the content of a file in a module. This idea is fully used in Flibs [START_REF] Markus | FLIBS -A collection of Fortran modules[END_REF]. However inclusion of file is considered as dirty programing since modules are available, but in this particular case it seems to be the only way. We emphasize that linked list once filled are transferred into an allocatable array which is more relevant from a computational point of view. -polymorphism :

Fortran9x allows some static polymorphism mechanism through the use of generic function interfaces.

The main drawback of this mechanism is that the choice of the real function is done regarding to the type of the arguments. Therefore it is irrelevant if the prototype (e.g. arguments) are similar. With Fortran200x norm comes the possibility to have pointer on procedures and any procedure with the same prototype can be pointed at by a field of a data structure. Then at run-time the pointer can be set imitating a kind of dynamic polymorphism. This is particularly interesting to write loop on objects without checks on type. Contact detection is a typical example, a first generic rough detection is made (using bounding box for example) and then a fine detection depending on the type of the two contactors needs to be performed. A pointer can be set to select the fine detection to use instead of having to go through a tedious selection.

Generating a Python interface

Plugging Fortran code into Python is usually seen as an awkward problem, despite f2py which aims to automatically generate a python interface to Fortran source code. Unfortunately f2py needs to compile some Fortran code and indeed to determine which compiler and version of the compiler is available ; which makes it system dependent and may need to modify some python script located in the system. Furthermore it is impossible to add docstrings (Python documentation) to the generated module. Therefore we have decided to use a new feature of the Fortran 200x norm which allows to automatically bind a Fortran routine in C through the use of the ISO_C_BINDING module. Adding a header file, which describes the prototypes of the routines, allows to use SWIG [START_REF] Beazley | Automated scientific software scripting with SWIG[END_REF], an automatic code conversion tool.

Combining this approach with the use of doxygen permits to generate the docstrings.

3 Main features

Modeling

LMGC90 offers a framework to describe complex 2D/3D multi-body systems :

-Shape of objects : described by simple convex primitive (disk, polygon, sphere, polyhedron, etc), cluster of primitives or general triangulated surface. Contact detection is performed for any combination of primitives. -Bulk behavior of objects may be rigid or deformable (linear : elastic or non linear : hyper-elastic, viscous, plastic, etc) by means of the finite element method. Behavior parameters may depend on external fields. Thermal effects and other physics (e.g. fluid dynamics) can also be modeled.

An external natural element method will be soon available. -Interaction laws : a large set of laws is available, textite.g. frictional contact, cohesion (capillarity, damage, brittle, etc), wire, rod. Thermal effects at contact may be taken into account.

Thanks to multiple discretizations of the same object, physic couplings can be performed at different scales through interpolation/projection methods.

Preprocessing tools are available to automatically generate granular samples, masonry structures, etc. Importing meshed geometry is also possible. See [START_REF] Martin | Conception d'un outil adapté à la mise en données des systèmes discrets, 10éme Colloque National en Calcul des Structures[END_REF] Once models are loaded in LMGC90 various possibilities are offered to check the validity of the model and eventually to correct it (e.g. log, visualization).

Analysis

LMGC90 multi-contact modeling strategy is mainly based on the Non Smooth Contact Dynamics method : non smooth dynamics framework, implicit time integration, implicit contact solvers (NLGS, PCG, etc). See [START_REF] Radjai | Discrete Numerical Modeling of Granular Materials[END_REF] for details on this method. Python supervisor is very useful to finely drive the analysis.

Thanks to the modular architecture other strategies are or may also be implemented (molecular dynamics strategy, explicit time integration, quasi-static time evolution, etc).

Applications with a large number of interacting bodies are reachable through parallel computing (Shared or distributed memory).

A non stationary thermal solver is available.

During computation it is possible to check the relevance of the analysis through some global indicators (convergence norm, quality of interaction laws computation, etc).

Post-processing

LMGC90 generates two categories of post-processing files of its simulation results :

-xml-vtk visualization files. These files describe the state of objects with various physical nodal fields, contact networks, Gauss points with their values, etc. Visualization may be performed with any tool supporting xml-vtk format as Paraview or Mayavi. -multi column text files. These files contain the result of analyses performed on all or part of the studied samples as contact orientation distribution, total dissipated energy, etc. They are suitable for any kind of plotting tool (xmgrace, MatPlotLib, gnuplot, etc).

Through the Python interface its also possible to access to various data which can be analyzed using built-in Python functions.

User Interface

LMGC90 is not integrated into a graphical environment and is usually driven by a Python script. Moreover it provides access to LMGC90 database, which offers the possibility to manage finely a simulation or couple with an external code.

A Python preprocessor aims at defining geometries, material properties, numerical modeling options, boundary conditions, etc. A typical use of LMGC90 concerns the study of the rheology of granular materials. As an example FIGURE 3 shows two types of results concerning the deformation of a sample made of 40,000 polyhedra (8 faces). On the left a detail of the packing is given ; the grey scale corresponds to the coordination number. On the right the contact network is represented ; line thickness is proportional to the force. The strong network is in grey and the weak in red.

An open platform

FIGURE 3 -Tri-axial compression of a polyhedral sample [13]

Basic features help with studying influence of shapes, interaction laws and bulk physics on the behavior of a collection of objects.

Masonry structures

A typical example is the stability study of ancient monument as Nîmes arena. On FIGURE 4 block geometry details of the and computed pressure into the joints due to dead load are given. This example was built using the Roman's conception rules. This kind of modeling allows assessing the stability and the

Modeling Fracture

LMGC90 allows to model fracture in various media. FIGURE 5 shows (left) fracture of an heterogenous media under horizontal traction load [START_REF] Perales | A NonSmooth Contact Dynamics-based multi-domain solver. Code coupling (Xper) and application to fracture[END_REF] and (right) the study of stability of a rock mass under self weight load [START_REF] Rafiee | The Non-Smooth Contact Dynamics method applied to the mechanical simulation of a jointed rock mass[END_REF]. Using a Frictional Cohesive Zone Model, fracture can be modeled, at microscopic or meso- Considering rock mass as a fractured media, avalanches on natural or mining slopes, stability tunnels, etc can be studied.

Multiple physics couplings

LMGC90 allows to consider various physics at different scales such as thermal coupling [START_REF] Radjai | Discrete Numerical Modeling of Granular Materials[END_REF], fluid particle interaction [START_REF] Martin | Ecoulement confiné d'un matériau granulaire en interaction avec un gaz, application à la relocalisation du combustible nucléaire[END_REF][START_REF] Topin | Micro-rheology of dense particulate flows : Application to immersed avalanches[END_REF], electrical conductivity [START_REF] Renouf | Coupling electrical and mechanical effects in discrete element simulations[END_REF]. FIGURE 5 shows (left) Sedimentation of thin particles in gaz and (right) an immersed avalanche of a loose granular assembly in a small closed box.

Conclusion

Started more than 10 years ago, the LMGC90 project is becoming more and more active from a research or application point of view. The nowadays challenges are multi-physics and multi-representation either to perform the switch of an object models (rigid -deformable) or to reduce-refine a part of a structure made of several objects.

The price to pay is a constant evolution of the overall software architecture and an improvement of the core technology.

Fortunately the Open-Source world provides a bunch of impressing software libraries that can be used. One aim of the Saladyn project is to couple LMGC90, Siconos and Code_Aster software through the Salome platform in order to take advantages of all these tools.

FIGURE 1 -

 1 FIGURE 1 -LMGC90 modules

FIGURE 2 -

 2 FIGURE 2 -Architecture of lmgc90

FIGURE 4 -

 4 FIGURE 4 -Modeling of Nîmes arena. Left the geometry. Right the pressure between blocks once at rest

FIGURE 5 -

 5 FIGURE 5 -fracture modeling examples

FIGURE 6 -

 6 FIGURE 6 -coupling modeling examples

  Even if Python allows to customized simulation, advanced developments benefit from an in Core For-tran90 implementation, which needs to have access to the sources which are open under CECILL license (textiti.e. GPL). This kind of license is particularly relevant to convince researchers to capitalize their scientific developments.

	4 Examples of Application
	4.1 Granular Material from rheology to structure
	Such an open platform enables coupling capabilities with external software (Finite Element code :
	Code_Aster, Pelicans) and libraries (Physical Models : MatLib, Linear Algebra : Lapack, Contact de-
	tection : Rapid, etc.). Furthermore the LMGC90 developer team is part of the Saladyn project (funded
	by ANR) and the Degrip project (funded by Oseo/FEDER).
	LMGC90 relies on collaborative programming tool (subversion & trac) :
	https://subver.lmgc.univ-montp2.fr/trac_LMGC90v2

Acknowledgements : the authors want to thanks all the LMGC90's contributing people in particular : G. Saussine, B. Chetouane, F. Perales, R. Perales, A. Rafiee, M. Vinches, M. Schryve, F. Kuss, H. Baudriller, C Silvani, J.J. Moreau, F. Radjai. The project is currently funded by ANR (Saladyn Project) and OSEO-FEDER (Degrip Project).

Références


Flavien Choulet
email: flavien.choulet@univ-orleans.fr

Michel Faure

Olivier Fabbri

Patrick Monié

Relationships between magmatism and extension along the Autun -La Serre fault system in the Variscan Belt of the eastern French Massif Central

Keywords: Variscan Belt, Late Carboniferous shear zones, synkinematic granite, 40 Ar-39 Ar dating, electron microprobe monazite dating, quartz c-axis, French Massif Central

The ENE-WSW Autun Shear Zone, in the northeastern part of the French Massif Central has been interpreted previously as a dextral wrench fault. New field observations and microstructural analyses document a NE-SW stretching lineation that indicates normal dextral motions along this shear zone. Further east, similar structures are observed along the La Serre Shear Zone. In both areas, a strain gradient from leucogranites with a weak preferred orientation to highly sheared mylonites supports a continuous Autun-La Serre fault system. Microstructural observations and shape and lattice preferred orientation document hightemperature deformation and magmatic fabrics in the Autun and La Serre granites, whereas low-to intermediate-temperature fabrics characterize the mylonitic granite. Electron microprobe monazite geochronology of the Autun and La Serre granites yields a ca. 320 Ma age for pluton emplacement, while mica 40 Ar-39 Ar datings of the Autun granite yield plateau ages from 305 to 300 Ma. The ca. 300 Ma 40 Ar-39 Ar ages, obtained on micas from Autun and La Serre mylonites, indicate the time of the mylonitization. The ca. 15 Ma time gap between pluton emplacement and deformation along the Autun-La Serre fault system argue against a synkinematic pluton emplacement during late-orogenic to post-orogenic extension of the Variscan Belt. A ductile to brittle continuum of deformation is observed along the shear zone, with Lower Permian brittle faults controlling the development of sedimentary basins. These results suggest a two-stage Late Carboniferous extension in the northeastern French Massif

Central, with regional crustal melting and emplacement of the Autun and La Serre leucogranites around 320 Ma, followed, at 305-295 Ma, by ductile shearing, normal brittle faulting, and subsequent exhumation along the Autun -La Serre transtensional fault system.

Introduction

Continental collision leads to the thrusting of two lithospheric plates and a subsequent crustal thickening during orogen formation. Once the lithostatic and compressional strengths are no longer balanced, the orogen becomes unstable and collapses (e.g. [START_REF] Malavieille | Kinematics of compressional and extensional ductile shearing deformation in a metamorphic core complex of the northeastern Basin and Range[END_REF][START_REF] Dewey | Extensionnal collapse of orogens[END_REF]). The gravitational collapse of the thickened crust is accommodated by extension and is characterized by thermal relaxation inducing partial melting of the continental crust [START_REF] England | Some thermal and tectonic models for crustal melting in continental collision zones[END_REF]. S-type magmatism, granite-gneiss domes and normal shear zones document syn to post-orogenic extension [START_REF] Reynolds | Evidence for large-scale transport on the Bullard detachment fault, west-central Arizona[END_REF][START_REF] Mcclay | Collapse of the Caledonian orogen and the Old Red Sandstones[END_REF][START_REF] Norton | Late Caledonide extension in western Norway: a response to extreme crustal thickening[END_REF]).

Paleozoic orogen interpreted as the result of the collision between Laurussia, Gondwana, and several intermediate microcontinents such as Armorica or Avalonia [START_REF] Matte | Tectonics and plate tectonics model for the Variscan Belt of Europe[END_REF]2001;[START_REF] Franke | Variscan plate tectonics in Central Europe. Current ideas and open questions[END_REF]). In the French Massif central, one of the main pieces of the Variscan orogen, successive phases of nappe stacking in Devonian and Early Carboniferous times [START_REF] Burg | A cross-section through the French Massif Central and the scope of its Variscan geodynamic evolution[END_REF][START_REF] Ledru | Où sont les nappes dans le Massif Central français?[END_REF]) led to an important thickening of the crust, and were followed by an important episode of crustal melting [START_REF] Duthou | Palaeozoic granitoids from the French Massif Central: age and origin studied by 87 Rb-87 Sr system[END_REF]. During the Late Carboniferous, the collapse of the Variscan Belt (Ménard and Molnar 1988;[START_REF] Burg | Syn-to post-thickening extension in the Variscan Belt of Western Europe: Modes and structural consequences[END_REF][START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF]) generated normal brittle faults [START_REF] Echtler | Extensional tectonics, basement uplift and Stephano-Permian collapse basin in a late Variscan metamorphic core complex (Montagne Noire, Southern Massif Central)[END_REF][START_REF] Faure | Sur la succession des épisodes extensifs au cours du dés-épaississement carbonifère du Massif Central français[END_REF], normal ductile shear zones [START_REF] Mattauer | Failles normales ductiles et grands chevauchements. Une nouvelle analogie entre l'Himalaya et la chaîne hercynienne du Massif central français[END_REF][START_REF] Echtler | Extensional tectonics, basement uplift and Stephano-Permian collapse basin in a late Variscan metamorphic core complex (Montagne Noire, Southern Massif Central)[END_REF]), synkinematic granitoids [START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Westphalian leucogranite in the Variscan Belt of the Northwest Massif Central, France[END_REF][START_REF] Talbot | Emplacement in an extensional setting of the Mont Lozère-Borne granitic complex (SE France) inferred from comprehensive AMS, structural and gravity studies[END_REF][START_REF] Joly | Gravity inversion, AMS and geochronological investigations of syntectonic granitic plutons in the southern part of the Variscan French Massif Central[END_REF], and granite-gneiss domes [START_REF] Ledru | The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution[END_REF]. Although the timing of these events is relatively well established [START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF], the connection between magmatism and extensional tectonics is not always clear. A common idea is that most of the plutons are synkinematic bodies emplaced along normal or strike-slip faults [START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Westphalian leucogranite in the Variscan Belt of the Northwest Massif Central, France[END_REF][START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF].

This study deals with extensional structures documented by field observations, microstructural analysis, and geochronology of granitoids and mylonites. We focus on the Morvan and La Serre horsts in the northeastern part of the French Massif Central, where Stype granites, ductile shear zones, and syn-sedimentary faults are exposed. We propose an interpretation of the late orogenic to post-orogenic evolution of this segment of the Variscan Belt. We shall use the classical chronostratigraphic stages of Western Europe for the Late Paleozoic period, with Namurian corresponding to Late Mississipian -Early Pennsylvanian , Westphalian to Middle Pennsylvanian (313-307 Ma), Stephanian to Late Pennsylvanian , Autunian to Late Pennsylvanian -Middle Cisuralian , and Saxonian to Late Cisuralian -Middle Guadalupian (276-263 Ma) [START_REF] Ogg | The Concise Geologic Time Scale[END_REF]).

Geological outline

2.1. The French Massif Central in the Variscan Belt Two types of scenario have been proposed to account for the geodynamical evolution of the Variscan Belt of Western Europe. The first type is monocyclic and is based on a continuous Paleozoic convergence between Gondwana and Laurussia [START_REF] Matte | Accretionary history and crustal evolution of the Variscan Belt in western Europe[END_REF][START_REF] Lardeaux | The Variscan Drench Massif Central -a new addition to the ultra-high pressure metamorphic "club": exhumation processes and geodynamic consequences[END_REF]. Alternatively, a polycyclic evolution with two successive orogenic episodes was proposed [START_REF] Pin | Variscan oceans: ages, origins and geodynamic implications inferred from geochemical and radiometric data[END_REF][START_REF] Faure | L'évolution polycyclique de la chaîne hercynienne[END_REF]. The reliability of the two scenarios is extensively discussed in several papers (e.g. [START_REF] Faure | Paleozoic tectonic evolution of medio-europa from the example of the French massif central and massif armoricain[END_REF]) and will not be addressed here. In this contribution, the polycyclic scenario is retained. In this model, the first cycle resulted from the Silurian north-directed subduction of the ocean that separated Gondwana and Armorica.

After an earlier high-pressure event dated at ca. 415 Ma [START_REF] Pin | Ages des episodes de métamorphisme paléozoïques dans le Massif central et le Massif armoricain[END_REF]Lardeaux et al., 2001), crustal nappes were stacked towards the SW, then partly migmatized, and finally exhumed during the Devonian around 390-380 Ma [START_REF] Floc | La série métamorphique du Limousin central[END_REF][START_REF] Quenardel | Palaeozoic evolution of the Plateau d'Aigurande (NW Massif Central, France)[END_REF][START_REF] Roig | La tectonique cisaillante polyphasée du S. Limousin (Massif central français) et son interprétation dans un modèle d'évolution polycyclique de la chaîne hercynienne[END_REF][START_REF] Faure | Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain[END_REF]). The second cycle was related to the closure of the Rheic Ocean that initially separated Laurussia and the Armorica microplate [START_REF] Faure | L'évolution polycyclique de la chaîne hercynienne[END_REF]. A Devonian to Early Carboniferous calk-alcaline magmatic suite (Pin et al. 1982) and the Devonian Brévenne ophiolite [START_REF] Leloix | Hercynian polyphase tectonics in north-east French Massif Central: the closure of the Brévenne Devonian-Dinantian rift[END_REF]) are interpreted as remnants of a magmatic arc and a back-arc basin related to the southward subduction of the Rheic Ocean [START_REF] Faure | L'évolution polycyclique de la chaîne hercynienne[END_REF].

During mid-Carboniferous times (Visean), the southern part of the Massif Central experienced top-to-the south nappe stacking, while synorogenic extension prevailed in the north. Late Visean magmatic series, locally called "Tufs Anthracifères", postdate the mid-Carboniferous events [START_REF] Faure | Late Visean thermal event in the northern part of the French Massif central: new 40 Ar/ 39 Ar and Rb-Sr isotopic constraints on the Hercynian syn-orogenic extension[END_REF]. The Late orogenic stage is characterized by two successive events [START_REF] Faure | Sur la succession des épisodes extensifs au cours du dés-épaississement carbonifère du Massif Central français[END_REF][START_REF] Burg | Syn-to post-thickening extension in the Variscan Belt of Western Europe: Modes and structural consequences[END_REF][START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF]): a Namurian to Westphalian NW-SE extension characterized by leucogranite emplacement [START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Westphalian leucogranite in the Variscan Belt of the Northwest Massif Central, France[END_REF], followed by a Stephanian to Permian NE-SW extension characterized by brittle normal faulting and formation of coal-bearing half-grabens [START_REF] Arthaud | Late Paleozoic strike-slip faulting in southern Europe and northern Africa: Result of a right-lateral shear zone between the Appalachians and the Urals[END_REF][START_REF] Echtler | Extensional tectonics, basement uplift and Stephano-Permian collapse basin in a late Variscan metamorphic core complex (Montagne Noire, Southern Massif Central)[END_REF][START_REF] Echtler | Extensional tectonics, basement uplift and Stephano-Permian collapse basin in a late Variscan metamorphic core complex (Montagne Noire, Southern Massif Central)[END_REF]). Whatever the geodynamic scenario (monocyclic or polycyclic), the Late Carboniferous syn-orogenic to post-orogenic extension is widely accepted,.

The northeastern French Massif Central

The study area is located in the northeastern French Massif Central, also called Morvan area, which forms a horst between the Cenozoic Limagne and Bresse grabens (Fig. 1). Paleozoic metamorphic units are unconformably overlain by Devonian to Early Carboniferous sedimentary and magmatic rocks [START_REF] Delfour | Données lithostratigraphiques et géochimiques sur le Dévono-Dinantien de la partie sud du faisceau du Morvan (nord-est du Massif Central français)[END_REF][START_REF] Faure | L'évolution polycyclique de la chaîne hercynienne[END_REF]) and are crosscut by Carboniferous magmatic intrusions with varied geochemical characteristics [START_REF] Rolin | Décrochements intracrustaux et intrusions granitiques carbonifères dans le Morvan (Massif central français)[END_REF]. The last manifestation of this magmatic event is the emplacement of peraluminous plutons such as the Autun granite [START_REF] Chévremont | Notice explicative de la carte géologique d' Epinac-Les-Mines au 1:50000, Bureau de recherches géologiques et minières, Orléans Cocherie A, Albarède F[END_REF]Fig. 2a), located close to ductile shear zones in the Autun and Avallon areas [START_REF] Rolin | Décrochements intracrustaux et intrusions granitiques carbonifères dans le Morvan (Massif central français)[END_REF]. Late orogenic brittle wrench or normal faults controlled the development of the Stephanian to Permian coal-bearing half-grabens or pull-apart basins such as the Autun, Epinac or Le Creusot basins [START_REF] Marteau | Le bassin permo-carbonifère d'Autun. Stratigraphie, sédimentologie et aspects structuraux[END_REF][START_REF] Vallé | Les marqueurs de la tectonique synsédimenatire et syndiagénétique dans le bassin stéphanien à régime cisaillant de Blanzy-Montceau (Massif Central, France)[END_REF]; Fig. 1). These intramontane basins are characterized by a terrigeneous sedimentation, which started during the Late Stephanian and remained active during the Permian [START_REF] Courel | Histoire tardi-hercynienne et couverture méso-cénozoïque[END_REF]. Stephanian to Permian high-K acidic volcanism is also reported [START_REF] Campy | Le volcanisme acide de l'Ouest Morvan dans son cadre géologique: caractérisation géochimique, structurale et chronologique de mise en place[END_REF][START_REF] Chévremont | Notice explicative de la carte géologique d' Epinac-Les-Mines au 1:50000, Bureau de recherches géologiques et minières, Orléans Cocherie A, Albarède F[END_REF].

East of the Oligocene Bresse graben, and close to the northern end of the Jura foldand-thrust belt, the La Serre horst displays structures similar to those of the Autun area (Fig. 3), with a leucogranitic pluton separated from a Permian sedimentary basin by a ductile shear zone and a brittle fault system [START_REF] Coromina | Late Palaeozoic NE-SW ductile-brittle extension in the La Serre horst, eastern France[END_REF].

3. Petrography and structure of the studied rocks

Field data

In the Autun area (Fig. 2a), a N70°E-trending, 500 m wide, mylonitic belt separates Variscan migmatites and granites to the south from a Stephanian to Autunian coal basin to the north [START_REF] Delfour | Données lithostratigraphiques et géochimiques sur le Dévono-Dinantien de la partie sud du faisceau du Morvan (nord-est du Massif Central français)[END_REF][START_REF] Rolin | Décrochements intracrustaux et intrusions granitiques carbonifères dans le Morvan (Massif central français)[END_REF]. The Autun leucogranite (Fig. 4a) intruded pre-Carboniferous gneisses and migmatites. A porphyritic facies of this leucogranite can be observed in some places. At the hand sample scale, this granite locally shows a planar preferred orientation of biotite and muscovite, and a linear preferred orientation, indicated by K-feldspar and biotite aggregates. However, given the scarcity of outcrops, it is difficult to draw a structural map of the whole pluton (Fig. 2a). To the north, the granitic rocks are more deformed, and the magmatic foliation is reoriented and changed to a tectonic foliation (Fig 2a;b). This change suggests that either the deformation might have occurred during the last stages of pluton emplacement, or that the granitic pluton experienced a post-solidus ductile deformation along the Autun fault. The mylonitization affects all the Variscan rocks, which then display a N60°E to N80°E striking foliation that dips 45° to 80° to the north; a stretching lineation trends N45°E and plunges to the NE (Fig. 2c). Macroscopic shear criteria indicate a top-to-the-NE motion. Shearing was oblique combining strike-slip and dip-slip components (Fig. 4b), not in agreement with previous results on the Autun Shear Zone for which a pure dextral strike-slip faulting was postulated [START_REF] Rolin | Décrochements intracrustaux et intrusions granitiques carbonifères dans le Morvan (Massif central français)[END_REF]. North of the shear zone, the mylonites are unconformably overlain by Permian deposits. However, north of Morlet (Fig. 2a), the contact between mylonites to the south and gneiss to the north is a brittle fault hidden westward beneath onlapping Permian deposits (Fig. 2b). Cataclastic mylonitic granites and silicified tectonic breccia, exposed at the southern border of the sedimentary basin, indicate brittle deformation. In the northern part of the Autun basin, the Permian strata overlie

Stephanian strata [START_REF] Marteau | Le bassin permo-carbonifère d'Autun. Stratigraphie, sédimentologie et aspects structuraux[END_REF][START_REF] Chévremont | Notice explicative de la carte géologique d' Epinac-Les-Mines au 1:50000, Bureau de recherches géologiques et minières, Orléans Cocherie A, Albarède F[END_REF] The Autun basin has been described as either a half-graben bounded by normal faults, or a pull-apart basin associated with left-lateral faults [START_REF] Marteau | Le bassin permo-carbonifère d'Autun. Stratigraphie, sédimentologie et aspects structuraux[END_REF]. The continental deposits composed of conglomerates and sandstones interlayerd with volcanic deposits [START_REF] Campy | Le volcanisme acide de l'Ouest Morvan dans son cadre géologique: caractérisation géochimique, structurale et chronologique de mise en place[END_REF][START_REF] Chévremont | Notice explicative de la carte géologique d' Epinac-Les-Mines au 1:50000, Bureau de recherches géologiques et minières, Orléans Cocherie A, Albarède F[END_REF]) yielded an Autunian to Saxonian flora described by [START_REF] Bergeron | Etude géologique du Massif ancien situé au Sud du Plateau Central[END_REF] who defined the Autunian stratotype. Sedimentary filling of this intramontane basin [START_REF] Courel | Histoire tardi-hercynienne et couverture méso-cénozoïque[END_REF]) is coeval with the activity along the normal boundary fault. Similar structures are observed in the La Serre area (Fig 3a). A two-mica granite (Morre-Biot 1969) with a weak mica preferred orientation is progressively deformed along the N50°E-trending La Serre Shear Zone, characterized by a N50°E-striking and 60°-to 80°northwest-dipping mylonitic foliation and a N30°E-trending stretching lineation [START_REF] Coromina | Late Palaeozoic NE-SW ductile-brittle extension in the La Serre horst, eastern France[END_REF]Fig. 3b). In thin sections, perpendicular to the foliation and parallel to the lineation (XZ sections), sigmoidal muscovites, shear bands and asymmetric porphyroclasts indicate a top-to-the-NE shear sense. To the northwest, the mylonitic belt is limited by the N70°E-to N80°E striking, low angle La Serre Median Brittle Fault, marked by a silicified volcanic breccia [START_REF] Coromina | Late Palaeozoic NE-SW ductile-brittle extension in the La Serre horst, eastern France[END_REF]Fig 3c). This breccia is similar to the silicified breccia exposed along the Autun shear zone. North of the La Serre Median Brittle Fault, Permian conglomerates and sandstones, with an Upper Autunian to Saxonian continental flora are about 500 m thick [START_REF] Campy | Le volcanisme acide de l'Ouest Morvan dans son cadre géologique: caractérisation géochimique, structurale et chronologique de mise en place[END_REF]. The La Serre Median Brittle Fault is postdated by flat lying deposits, but it was probably moderately reactivated as a normal fault during the Oligocene extensional tectonics of the Bresse graben and as a reverse fault during Alpine shortening [START_REF] Coromina | Late Palaeozoic NE-SW ductile-brittle extension in the La Serre horst, eastern France[END_REF][START_REF] Madritsch | Interactions between thin-and thickskinned tectonics at the northwestern front of the Jura fold-and-thrust belt (eastern France)[END_REF].

Microstructures, shape and lattice preferred orientation analyses

In order to understand the mechanisms of deformation of the Autun and La Serre leucogranites, the rock fabric was studied by several methods. The evolution from a magmatic planar fabric to a tectonic foliation was investigated by relying on mica preferred orientation.

Since rock fabric is not well defined, especially in apparently undeformed granites, systematic manual measurements of the orientation of longitudinal sections of muscovite and biotite platelets were carried out in the three principal planes of the strain ellipsoid (XY, XZ and YZ). These planes were estimated from field observations. Manual measurements were also supported by an automatic method using the "SPO" software [START_REF] Launeau | Determination of fabric and strain ellipsoids from measured sectional ellipses -Implementation and applications[END_REF]. On thin section images, each mica was handled as an isolated grain (Fig. 5). The orientation of each grain was computed by using the intercept method, with measurements shown in rose diagrams (Fig. 5). There are no significant differences between manual and automatic procedures. Quartz lattice-preferred orientation (LPO) analysis provides information about deformation mechanisms, glide systems, and activation temperature. Quartz c-axis orientation (Fig. 6) was studied with an U-stage.

Magmatic microstructures

In the two samples of undeformed granite (AU16 and AU19, Fig. 5), a planar fabric can be inferred from the orientation of mica flakes along the XZ and YZ sections. A linear fabric visible along XY sections is better developed in sample AU16 than in sample AU19.

Coarse angular quartz grains show seriate to polygonal textures [START_REF] Moore | Descriptive terminology for the textures of rocks in granulite facies terrains[END_REF]. Quartz grains display a chessboard pattern of subgrain boundaries (Fig. 4c). Myrmekite is also observed (Fig. 4d) and could indicate a syn-magmatic deformation [START_REF] Hibbard | Deformation of incompletely crystallized magma systems: granitic gneisses and their tectonic implications[END_REF], although myrmekitic textures can also develop under solid-state conditions [START_REF] Simpson | Evidence for deformation-induced K-feldspar replacement by myrmekite[END_REF].

In samples from the undeformed Autun and La Serre granites (AU16, AU19, AU28, AU29 and SE03, Fig. 6), c-axis maxima in the XZ plane are observed at 0 to 45° from the X axis of the finite strain. Such a c-axis distribution is typical of prism <c> slip and is representative of high-temperature deformation mechanisms [START_REF] Mainprice | Dominant c-slip in naturally deformed quartz: implications for dramatic plastic softening at high temperature[END_REF]).

Nevertheless, the chessboard textures observed in the two plutons indicate either a combined basal <a> and prism <c> slip [START_REF] Mainprice | Dominant c-slip in naturally deformed quartz: implications for dramatic plastic softening at high temperature[END_REF][START_REF] Stipp | The eastern Tonale fault zone: a "natural laboratory" for crystal plastic deformation of quartz over a temperature range from 250 to 750 °C[END_REF], or the transition from low-temperature to high-temperature quartz [START_REF] Kruhl | Prism-and basal-plane parallel subgrain boundaries in quartz: a microstructural geothermobarometer[END_REF]. Whatever the mechanism is, and considering that no fluids have caused a late recrystallization, microstructures in undeformed granite reveal high temperature of deformation of ca. 700°C, close to the magma crystallization [START_REF] Kruhl | Prism-and basal-plane parallel subgrain boundaries in quartz: a microstructural geothermobarometer[END_REF].

Low-temperature mylonite deformation

In the two samples of mylonitic granite (AU02 and AU20, Fig. 5), well-defined maxima in the rose diagram correspond to the rock planar fabric. The bimodal orientation of micas probably correlates with distinct shear bands and recrystallization of mica. In the mylonitic granite, microstructures are characteristic of ductile deformation with shear bands crosscutting a preexisting foliation (Fig. 4b). This geometry must be distinguished from the S-C structures, which have been originally defined for a synkinematic pluton where S and C surfaces are formed synchronously [START_REF] Berthé | Orthogneiss, mylonite and non coaxial déformation of granites: the example of the South Armorican Shear Zone[END_REF]. During mylonitization, the granite experienced grain size reduction; undulatory quartz grains with lobate boundaries indicate dynamic recrystallization during low-temperature deformation (Fig. 4e). Recrystallization of such fine quartz grains may be due to combined subgrain rotation and bulging recrystallizations [START_REF] Stipp | The eastern Tonale fault zone: a "natural laboratory" for crystal plastic deformation of quartz over a temperature range from 250 to 750 °C[END_REF]. The asymmetry of feldspar porphryroclasts with sigmatype recrystallization tails (Fig. 4f), sigmoidal micafish (Fig. 4g), quartz ribbons, and shear band geometry suggest a non-coaxial strain regime with a top-to-the-NE sense of shear (Fig. 4h, [START_REF] Passchier | Répartition des terres rares dans les roches volcaniques basiques dévono-dinantiennes du nord-est du Massif central[END_REF], in agreement with a normal-dextral sense of the shear.

In deformed granite samples (AU02, AU03, AU04, AU20, SE01 and SE02, Fig. 6), quartz c-axis measurements reveal an incomplete type I crossed griddle pattern [START_REF] Lister | The simulation of fabric development during plastic deformation and its application to quartztite: the influence of deformation history[END_REF]. Maxima around the main shortening direction axis (Z) indicate activity of basal <a> slip systems, and the dominant recrystallization mechanism is bulging recrystallization [START_REF] Stipp | The eastern Tonale fault zone: a "natural laboratory" for crystal plastic deformation of quartz over a temperature range from 250 to 750 °C[END_REF]. Submaxima distributed either close to the Y axis, or in between the Y and Z axes stem from prism <a> and rhomb <a> slips [START_REF] Bouchez | Plastic deformation of quartzites at low temperature in an area of natural strain gradient[END_REF], with dominant subgrain rotation recrystallization [START_REF] Stipp | The eastern Tonale fault zone: a "natural laboratory" for crystal plastic deformation of quartz over a temperature range from 250 to 750 °C[END_REF]. The coexistence of these slip systems may be due to different activation temperature; at ca. 300°C, basal <a> slip is dominant, whereas, at 500°C, prism <a> is more important [START_REF] Bouchez | Plastic deformation of quartzites at low temperature in an area of natural strain gradient[END_REF][START_REF] Stipp | The eastern Tonale fault zone: a "natural laboratory" for crystal plastic deformation of quartz over a temperature range from 250 to 750 °C[END_REF][START_REF] Passchier | Répartition des terres rares dans les roches volcaniques basiques dévono-dinantiennes du nord-est du Massif central[END_REF]) However, at low temperature, with an increasing strain, the slip sequence is basal <a>, followed by prism <a>, and finally rhomb <a> [START_REF] Passchier | Répartition des terres rares dans les roches volcaniques basiques dévono-dinantiennes du nord-est du Massif central[END_REF]. Since basal <a> slips are mainly observed in protomylonites, and both prism <a> and rhomb >a> slips are dominant in the most deformed mylonites (Fig. 6), the second alternative seems more likely.

The asymmetry of the c-axis maxima suggests a non-coaxial progressive deformation, under low to intermediate temperature, with a top-to-NE sense of shear in agreement with other kinematic indicators.

New geochronological data

To constrain the age of the granitoid emplacement and the time of the ductile deformation, monazite U-Th-Pb geochronology and micas 40 Ar/ 39 Ar dating have been carried out.

Electron microprobe (EPMA) monazite geochronology

The EPMA U-Th-Pb dating on monazite is a chemical method based on two main assumptions related to the mineral structure. (1) The common lead incorporated in monazite during crystallization is negligible compared to the amount of radiogenic lead [START_REF] Parrish | U-Pb dating of monazite and its application to geological problems[END_REF][START_REF] Cocherie | Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total Pb determination: implications for lead behaviour in monazite[END_REF], and (2) no radiogenic lead loss occurred since the system closure (considered as the crystallization time in magmatic rocks). These assumptions have been confirmed both by experimental studies, and by comparison of EPMA dating and conventional U/Pb isotopic methods [START_REF] Cocherie | Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total Pb determination: implications for lead behaviour in monazite[END_REF]2005;[START_REF] Montel | X-ray diffraction study of brabantitemonazite solid solutions[END_REF].

Moreover, altered domains with potential lead loss can be avoided with the EPMA resolution (2 µm) associated with Backscattered Electron (BSE) microscopy. This monazite EPMA method is well suited for the dating of leucogranite, as monazite is common in peraluminous granite (e.g. Bé [START_REF] Bé Mézème | Electron microprobe monazite geochronology of magmatic events: Examples from Variscan migmatites and granitoids, Massif Central, France[END_REF].

In-situ monazite grains were analyzed with a Cameca SX 50 electron microprobe, cooperated by ITSO and BRGM, following the analytical procedure described in [START_REF] Cocherie | Geochronology of polygenetic monazites constrained by in situ electron microprobe Th-U-total Pb determination: implications for lead behaviour in monazite[END_REF]. Acceleration voltage is 20 kV, beam current is 200 nA, and U, Th, Pb absolute errors are 105 ppm, 130 ppm and 110 ppm, respectively. The U-Th-Pb age calculations were done by using the "EPMA dating" program written by [START_REF] Pommier | EPMA Dating User's Manual: Age Calculation from Electron Probe Microanalyser Measurements of U-Th-Pb[END_REF]. The age calculated for each microprobe point analysis is rejected if out of the confidence range. The sorted results are computed and plotted with the ISOPLOT program [START_REF] Ludwig | Users manual for ISOPLOT/EX, version 2. A geochronological toolkit for Microsoft Excel -Berkeley Geochronology Center[END_REF][START_REF] Ludwig | User's manual for Isoplot 3[END_REF]. Due to a relatively large range in Th/U ratios, the isochron method of Cocherie and Albarede (2001) was used to produce the U/Pb vs Th/Pb diagrams reported in Figure 6. The following parameters were extracted from the diagram and used to compute the best fit line age calculation for each sample: (1) Th-Pb age (intercept with Th/Pb axis) and U-Pb age (intercept with U/Pb axis) and their respective errors (2σ); (2) slope and error of the best fit line, X-Y coordinates of the centroid of the best fit line. The results are statistically acceptable if the following three conditions are fullfilled. : (1) The theoretical Mean Square Weight Deviation (MSWD) is above the calculated MSWD, (2) the theoretical isochron crosscuts the envelope error of each analysis and (3) the intercept ages are similar within the error margin.

In the studied samples, monazite grains are included in biotite and predate the crystallization of the mica (Fig 7a). BSE images (Fig. 7b) of monazite in the Autun undeformed leucogranite (AU16, N46°55'55''; E4°19'22'') show medium-sized euhedral to sub-euhedral grains (50 to 100 µm) with no significant chemical zoning. The data scatter is relatively good and an isochron age of 318±7 Ma has been calculated on 8 grains (Fig. 7b).

The MSWD is slightly higher than the theoretical value (1.5 vs. 1.4). BSE images of monazite in mylonitic Autun leucogranites sample (AU26, N46°57'00"; 4°21'35") show euhedral grains not deformed during the ductile event and still exhibiting typical growth zoning (Fig. 7c). Most of the analyses are clustered but, since some analyses show a high U/Pb ratio, a best fit line can be drawn, and an isochron age of 321±3 Ma has been calculated on 5 grains (Fig. 7c). The intercept ages (U-Pb age 323±6 Ma and Th-Pb age: 306±36 Ma) are similar within errors. The MSWD is largely below the theorical value (0.23 vs. 1.30), a statistically meaningful result. BSE images of monazite in the La Serre granite (SE03, N47°10'20"; E5°33'24") show medium-sized sub-euhedral to anhedral grains (Fig. 7d). The data are widely spread in the isochron diagram and a best fit line can be drawn. The calculation on 6 grains provides an isochron age of 317±5 Ma (Fig. 7d). Although the theoretical isochron is at the limit of the error envelope of the best fit line, the results are acceptable as the intercept ages (U-Pb age 290±30 Ma and Th-Pb age: 338±25 Ma) are similar within error magin, and the calculated MSWD is slightly lower than its theoretical value (1.3 vs. 1.38).

Deformation and cooling age of the Autun and the La Serre granites

In order to date the ductile faulting and to constrain the relationships between magmatism and deformation, we performed 40 Ar/ 39 Ar dating on mylonites from the two shear zones and on the Autun granite, and assumed that muscovite and biotite ages from these samples would provide constraints on their cooling history in the 300°C-450°C range [START_REF] Harrison | Diffusion of 40 Ar in biotite : temperature, pessure and compositional effects[END_REF][START_REF] Harrison | Diffusion of 40 Ar in muscovite[END_REF][START_REF] Hames | An empirical evaluation of the argon diffusion in muscovite[END_REF]. As these cooling temperatures are similar to those deduced from LPO measurements in mylonitic rocks, the 40 Ar/ 39 Ar method should give a reliable estimate of the age of deformation and an estimate of the cooling rate of the granite. Details about the analytical procedure for laser probe 40 Ar/ 39 Ar dating can be found in [START_REF] York | 40 Ar/ 39 Ar dating of terrestrial minerals with a continuous laser[END_REF], [START_REF] Dalrymple | The GLM continuous laser system for. 40 Ar/ 39 Ar dating: description and performance characteristics[END_REF], [START_REF] Mcdougall | Geochronology and Thermochronology by the 40 Ar/ 39 Ar method[END_REF], [START_REF] Monié | Cooling and exhumation of the western Betic Cordilleras, 40Ar/39Ar thermochronological constraints on a collapsed terrane[END_REF] and [START_REF] Monié | Coeval blueschist exhumation along thousands of km : implications for subduction channel processes[END_REF]. Each sample was first crushed and micas were separated under a binocular from the 0.5-07 mm size fraction. After ultrasonic cleaning, micas were enveloped in aluminium foils and irradiated at McMaster (Canada) together with several aliquots of the MMhb1 monitor amphibole (520.4 ± 1.7 Ma; [START_REF] Samson | Calibration of the interlaboratory 40 Ar-39 Ar dating standard, MMhb-1[END_REF]. After irradiation, the micas and standards were placed on a copper holder inside the sample chamber and heated at 150°C under ultrahigh vacuum.

Step-heating degassing of individual grains was performed using a continuous CO 2 laser until complete fusion of the mineral. For each step, the released gas was cleaned on getters and then introduced in a MAP 215-250 mass spectrometer for analysis of the isotopic composition, estimated by regression on 15 runs. Extraction, cleaning, and analysing processes involve 1, 2 and 8 minutes respectively. System blanks were realized every three experiments. Depending on the samples, 9 to 17 steps were performed. Individual age (Tab. 1) was calculated after usual isotope corrections including blanks, mass discrimination, radioactive decay of 37 Ar and 39 Ar, and irradiation-induced mass interference.

For the MAP 215-250 mass spectrometer, a 40 Ar/ 36 Ar ratio of 285.0 ± 2.0 was used for mass discrimination of all analyses. The uncertainty on the J-factor was propagated in the calculation of the error on the total age of each sample, equivalent to a K-Ar age. Results are reported as classical 40 Ar/ 39 Ar age spectra (Fig. 8).

The term "plateau age" is defined as the segment of a spectrum, made of three consecutive steps, containing more than 70% of the total 39 Ar released, and whose ages overlap within two sigma errors [START_REF] Mcdougall | Geochronology and Thermochronology by the 40 Ar/ 39 Ar method[END_REF]. Ages are reported with a 1σ uncertainty. It is worth to mention that the Autun shear zone experienced an important fluid circulation that caused silicification, galena mineralization and secondary large muscovite flake crystallization, probably during late brittle faulting [START_REF] Delfour | Données lithostratigraphiques et géochimiques sur le Dévono-Dinantien de la partie sud du faisceau du Morvan (nord-est du Massif Central français)[END_REF]. Since this alteration event possibly influenced the argon isotopic system, only unaltered samples were choosen.

Biotite from the inner part of the Autun granitic massif shows a discordant spectrum with ages varying from 297 Ma to 308 Ma and a plateau age of 306.4 ± 4 Ma calculated on the last 70 % of 39 Ar released (sample AU29, N46°54'30"; E4°16'20", Fig. 8a). Muscovite from the Autun protomylonite (sample AU17, N46°56'9"; E4°19'10", Fig. 8b) displays a slightly discordant spectrum with ages ranging from 261 to 355 Ma. More than 80 % of the total 39 Ar released gives a plateau age of 303.9 ± 3 Ma in the intermediate portion of the spectrum.

Biotite from the Autun granite (AU16, N46°55'55''; E4°19'22'', Fig. 8c) has a discordant spectrum with evidence of argon loss at the beginning of degassing and excess argon at the end, resulting in ages varying from 133 Ma to 310 Ma. A plateau age of 299.6 ± 3 Ma has been calculated on more than 75 % of 39 Ar released in the intermediate portion of the spectrum. Muscovite from sample AU16 (Fig. 8d) shows a relatively flat spectrum with values ranging mainly between 291 Ma and 301 Ma, with the exception of the first step related to argon bound to the mica surface. A plateau age of 299.8 ± 3 Ma is calculated for more than 80 % of the total 39 Ar released.

Biotite from the Autun mylonite (AU15, N46°56'24''; E4°18'53'', Fig. 8e) displays a highly discordant spectrum with ages varying from 220 Ma to 303 Ma and no plateau age. It is likely that incipient chloritization is responsible for such a pattern, with combined effects of argon loss and neutron-induced 39 Ar recoil [START_REF] Turner | Possible effects of 39 Ar recoil in 40 Ar-39 Ar dating[END_REF][START_REF] Ruffet | Comparison of 40 Ar-39 Ar conventional and laser dating of biotites from the North Trégor Batholith[END_REF].

Muscovite from sample AU15 gives a much less discordant spectrum with young ages for low experimental temperatures (Fig. 8e). With the exception of the three first steps, the ages are bracketed between 295 and 304 Ma, and a plateau age of 299.8 ± 3.0 can be calculated on 95% of the total 39 Ar released.

Muscovite from the La Serre mylonite (sample SE01Ms, 47°11'19"; 5°31'27", Fig. 8f) displays a discordant spectrum ranging from 289 to 321 Ma, with argon excess released at low experimental temperature. However for 75 % of the total 39 Ar released, ages are bracketed between 296 Ma and 299 Ma and correspond to a plateau age of 298.2 ± 3 Ma. In this sample, the deformation is very strong, and biotite is often replaced by chlorite.

Discussion

Regional correlations of the Autun and La Serre Faults

The western and eastern continuations of Variscan shear zones in the northern French Massif Central remained speculative for a long time [START_REF] Arthaud | Late Paleozoic strike-slip faulting in southern Europe and northern Africa: Result of a right-lateral shear zone between the Appalachians and the Urals[END_REF]. [START_REF] Rolin | Décrochements intracrustaux et intrusions granitiques carbonifères dans le Morvan (Massif central français)[END_REF] considered the Autun Shear Zone as an Upper Devonian to Middle Carboniferous dextral wrench fault and, after restoring the sinistral offset of the Sillon Houiller Fault, correlated it with the Marche-Combrailles Fault (see also [START_REF] Lerouge | Chronologie des événements tectoniques dans le Nord-Ouest du Massif Central français et le Sud du bassin de Paris du Carbonifère inférieur au Plio-quaternaire[END_REF][START_REF] Rolin | Carte structurale du socle Varisque Vendée-Seuil du Poitou-Limousin[END_REF]. However, our new data and other recent 40 Ar-39 Ar dating in the La Marche area (Gebelin et al. 2007) indicate that the Marche-Combrailles and the Autun shear zones are two different shear zones. Recent geophysical studies also suggest that the eastern continuation of the Marche-Combrailles Fault is the Avallon Fault [START_REF] Edel | Structure et nature du socle anté-permien du Bassin de Paris d'après les données gravimétriques et magnétiques -Le problème de l'Anomalie Magnétique du Bassin de Paris (AMBP)[END_REF]; Fig. 9). The eastern continuation of the Autun Shear zone is less hypothetic, as shown by the similar structures observed along the La Serre Fault Zone. After restoring the E-W offset related to the Cenozoic opening of the Bresse graben, the two areas belong to the same fault system. However, vertical derivation of the Bouguer gravity anomaly map does not support a direct continuity between the two shear zones (Edel, unpublished data), but rather suggest that the Autun Shear Zone joins a fault zone parallel to the La Serre Shear Zone, located about 10 km to the north of the La Serre Shear Zone. The southwestern extension of the N65°E trending La Serre brittle Fault might be the fault that rims the Creusot Basin (Fig. 1). Presently, there is no evidence of the western continuation of the La Serre ductile shear zone to the Morvan area. The La Serre ductile shear zone may extend westward below the Creusot sedimentary basin. All these E-W to N65°E trending faults are crosscut by the La Serre Southern brittle Fault (Fig. 3), a segment of the NE-SW striking Sainte-Marie-Aux-Mines Fault. This sinistral fault hides the eastern extension of the Autun-La Serre fault system (Fig. 9). However, recent work on the Upper Rhine graben (Edel et al. 2007;[START_REF] Ustaszewski | Latest Pliocene to recent thick-skinned tectonics at the Upper Rhine Graben -Jura Mountains junction[END_REF]) and along the Rhine-Bresse Transfer Zone [START_REF] Madritsch | Interactions between thin-and thickskinned tectonics at the northwestern front of the Jura fold-and-thrust belt (eastern France)[END_REF]) indicate the presence beneath the Mesozoic sedimentary cover of a large set of NE-SW to E-W pre-Mesozoic steeply dipping faults that might represent the eastern continuations of the Autun-La Serre fault system (Fig. 9). These Variscan faults were locally reactivated by the Late Eocene-Oligocene extension in the Upper Rhine graben (Edel et al. 2007) or by the Pliocene shortening during formation of the Jura fold-and-thrust belt [START_REF] Giamboni | Plio-Pleistocene Transpressional Reactivation of Paleozoic and Paleogene Structures in the Rhine-Bresse transform Zone (northern Switzerland and eastern France)[END_REF]). The gentle curvature of the Autun La Serre Fault System, from an E-W trend, to the west, to an ENE-WSW trend, to the east, could be related to local block rotations either due to Late Variscan adjustments as suggested by Edel and Schulmann (2007) or by Oligocene extension in the Bresse graben, or due to Pliocene tectonics of the Jura Mountains.

Timing of the extensional processes

Microstructural observations, and shape and lattice preferred orientation analyses indicate several types of deformation. As deduced from quartz <c> axis measurements and chessboard pattern, the Autun and La Serre plutons underwent a high-temperature deformation, the mechanisms of which remains controversial; it could be either be due to prism <c> slip [START_REF] Mainprice | Dominant c-slip in naturally deformed quartz: implications for dramatic plastic softening at high temperature[END_REF] or grain growth and boundary migration [START_REF] Gapais | Quartz fabric transition in a cooling syntectonic granite (Hermitage massif, France)[END_REF] or α to β quartz transition [START_REF] Kruhl | Prism-and basal-plane parallel subgrain boundaries in quartz: a microstructural geothermobarometer[END_REF]. Whatever the mechanism is, deformation likely occurred during granite emplacement. Furthermore, low-to intermediate temperature shearing in protomylonites, mylonites and ultramylonites indicates a strain gradient from the pluton margin, where a tectonic fabric dominates, to the pluton core, where sub-solidus preferred orientation is prominent. Such a strain gradient could be due to localized deformation either during magma crystallization or under solid-state conditions after granite emplacement.

Monazite U-Th/Pb dating yields consistent ages at ca. 320-318 Ma (Namurian) for the Autun and La Serre granites and the Autun mylonitic granite, indicating that the U-Th-Pb system was not reset during deformation (Figs. 7 and 10). A modification of the chemistry of the monazite can occur in amphibolite facies shear zones [START_REF] Berger | Deformation of monazite in an amphibolitefacies shear zone[END_REF]) but, in the present case, the lack of core-and-mantle zoned grains and the preservation of growth zoning suggest that monazite grains did not recrystallize during mylonitization. Moreover, since the dated monazites are included in biotite, they predate the crystallization of the micas.

Crystallization of magmatic monazite in peraluminous systems occurs during granite emplacement [START_REF] Braun | Electron microprobe dating of monazite from high-grade gneisses and pegmatites of the Kerala Khondalite Belt, southern India[END_REF][START_REF] Förster | The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany, Part I: The monazite (Ce)-brabantite solid solution series[END_REF], at temperatures of about 900°C [START_REF] Cherniak | Pb diffusion in monazite: a combine RBS/SIMS study[END_REF]). Thus, the monazite U-Th/Pb ages mirror the crystallization age of the magma. The Namurian age assignment is significantly different from the Stephanian age previously proposed for the Autun granite [START_REF] Rolin | Décrochements intracrustaux et intrusions granitiques carbonifères dans le Morvan (Massif central français)[END_REF] and from the Devonian U-Pb (TIMS) age proposed for the La Serre granite [START_REF] Morre-Biot | Sur l'age absolu du granite de la Serre[END_REF]. The consistency between our new monazite U-Th/Pb ages and the petrographic and microstructural observations reveals that mylonite formed at the expense of the Autun granite. During the pluton emplacement, a poorly defined subsolidus fabric was acquired. However, the significance of this magmatic preferred orientation is not clear and might be due to magmatic processes in the magma chamber or to a combination of magma dynamics and regional tectonics [START_REF] Brun | Strain patterns of pluton emplacement en a crust undergoing non-coaxial deformation, Sierra Morena, Southern Spain[END_REF].

Whatever the deformation gradient obtained in the Autun and La Serre areas might be, the Late Carboniferous 40 Ar/ 39 Ar ages around 300 Ma show a regional consistency (Figs. 8 and10a). One interpretation could be that a Late Carboniferous thermal event reset the argon isotopic system, a phenomenon recognized for the Late Visean evolution in the central part of the French Massif Central [START_REF] Faure | Late Visean thermal event in the northern part of the French Massif central: new 40 Ar/ 39 Ar and Rb-Sr isotopic constraints on the Hercynian syn-orogenic extension[END_REF]. Although a Stephanian to Permian volcanism is recorded in the northeastern part of the French Massif Central [START_REF] Campy | Le volcanisme acide de l'Ouest Morvan dans son cadre géologique: caractérisation géochimique, structurale et chronologique de mise en place[END_REF][START_REF] Chévremont | Notice explicative de la carte géologique d' Epinac-Les-Mines au 1:50000, Bureau de recherches géologiques et minières, Orléans Cocherie A, Albarède F[END_REF], it seems to be too scattered and too small in volume to have caused such a large thermal overprint.

Protomylonites and undeformed granite exhibit 40 Ar/ 39 Ar spectra with plateau ages ranging from 305 to 300 Ma (Fig. 8), representing the time when the cooling path of the granite intersected the mica closure temperature. Due to their different closure temperatures, the concordance between muscovite and biotite 40 Ar/ 39 Ar ages indicates a fast cooling between 400°C and 300°C. The 40 Ar/ 39 Ar plateau age of undeformed rocks is statistically undistinguishable from those of mylonites (Figs. 8 and10a). The range of closure temperature of the two micas is similar to the temperature interval in which quartz lattice slip was activated. The Stephanian age thus likely corresponds to the time of the low-temperature noncoaxial ductile deformation experienced by the Autun and La Serre plutons. The age of this ductile deformation is furthermore constrained by the Lower Autunian (303 Ma to 290 Ma) sedimentary deposits of the Autun basin (Fig. 10). The earliest activity along the Autun-La Serre shear zone remains unknown, as 40 Ar-39 Ar dating only provides the age of the youngest ductile motion. Deformation the Autun-La Serre Fault System might have started before 305 Ma. However, several lines of evidence suggest that these granites are not syntectonic plutons. High-temperature post-solidus shearing could not be demonstrated in the analyzed rocks and only magmatic sub-solidus textures are observed in undeformed granites.

Moreover, the time gap of at least 15 myrs between granite emplacement and the last shear motion would imply a long-lived localized tectonic zone. As described in the geological outline section, such a time gap is not documented in the French Massif Central, where the Namurian -Westphalian NW-SE extension was followed by the Stephanian -Permian NE-SW extension [START_REF] Faure | Sur la succession des épisodes extensifs au cours du dés-épaississement carbonifère du Massif Central français[END_REF][START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF]. If a syntectonic granite had been emplaced in Namurian times, it would probably have recorded the NW-SE stretching associated with the widespread first extensional stage, [START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF][START_REF] Talbot | Pull-Apart emplacement of the Margeride granitic complex (French Massif Central). Implications for the late evolution of the Variscan orogen[END_REF][START_REF] Joly | Gravity inversion, AMS and geochronological investigations of syntectonic granitic plutons in the southern part of the Variscan French Massif Central[END_REF].

Our results demonstrate the existence of a Stephanian -Autunian NE-SW Autun-La Serre transtensional shear zone system. Its age is slightly younger than the Westphalian to

Stephanian Pilat detachement fault that shows similar mechanisms and structures [START_REF] Echtler | Extensional tectonics, basement uplift and Stephano-Permian collapse basin in a late Variscan metamorphic core complex (Montagne Noire, Southern Massif Central)[END_REF]). Although well documented [START_REF] Burg | Syn-to post-thickening extension in the Variscan Belt of Western Europe: Modes and structural consequences[END_REF][START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF]), the Stephanian -Permian NE-SW stretching must be distinguished from the NW-SE Namurian -Westphalian extension reported from the western part of the Massif Central [START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF]. Extensional structures are also documented beyond the Rhine Bresse transfer zone in the Vosges Mountains and Black Forest. Similar to the French Massif Central, the convergence of Gondwana and Laurussia has resulted in thrust tectonics and metamorphism in the Vosges Mountains [START_REF] Fluck | Le socle vosgien, segment de la chaîne varisque d'Europe : projet Vosges : état des connaissances, propositions d'axes de recherches[END_REF][START_REF] Rey | Evolutions contrastées dans les Vosges orientales: témoins d'un charriage dans la chaine varisque[END_REF][START_REF] Schulmann | Rapid burial and exhumation during orogeny: thickening and synconvergent exhumation of thermally weakened and thinned crust (Variscan orogen in western Europe)[END_REF], and in the Black Forest [START_REF] Wickert | Two-sided Variscan thrust tectonics in the Vosges Mountains, northeastern France[END_REF]Eisbacher et al., 1989;[START_REF] Echtler | Carboniferous convergence and subsequent crustal extension in the southern Schwarzwald (SW Germany)[END_REF]). The thickened crust experienced syn-orogenic to post-orogenic extension represented by detachment faults and syntectonic plutons, documented in the Vosges Mountains [START_REF] Rey | Middle and Late Carboniferous extension in the Variscan belt: structural and petrological evidence from the Vosges massif (eastern France)[END_REF][START_REF] Kratinova | Model of successive granite sheet emplacement in trantensionnal setting: Integrated microstructural and anisotropy of magnetic susceptibility study[END_REF], and in the Black Forest [START_REF] Krohe | Oblique crustal detachment in the Variscan Schwarzwald, southwestern Germany[END_REF]Eisbacher et al. 1989;[START_REF] Echtler | Carboniferous convergence and subsequent crustal extension in the southern Schwarzwald (SW Germany)[END_REF].

Implications for the thermal conditions during post-orogenic extension

As 40 Ar-39 Ar ages of undeformed and deformed granites exhibit similar plateau ages around 300 Ma and are ca. 15 myrs younger than the monazite U-Th-Pb ages of pluton emplacement, the contemporaneity between the emplacement of plutons and the oblique-slip displacement along the Autun-La Serre Shear Zone is ruled out. However, additional geochronological studies are required to solve the question of « the synkinematic granites ».

Furthermore, the time gap of 15 myrs between U-Th/Pb ages and 40 Ar-39 Ar ages of the granite is larger than ca. 5 myrs cooling times from 750°C (emplacement temperature) to 300°C, experimentally established for similarly-sized leucogranites [START_REF] Annen | Thermal evolution of leucogranites in extensional faults: implications for Miocene denudation rates in the Himalaya[END_REF][START_REF] Annen | Thermal evolution of leucogranites in extensional faults: implications for Miocene denudation rates in the Himalaya[END_REF]. We propose two different cooling paths (Fig. 10b). One two-stage cooling path is characterized by a "normal cooling" during the first 5 myrs, with a 100°C/Myr cooling rate, followed by a 10 myrs steady-state step around 300°C; in this case, 40 Ar-39 Ar ages of micas in undeformed granites should be close to U-Th/Pb ages and significantly older than age of mylonitization. An alternative cooling model could be a single step path, with continuous cooling from 800°C at 320 Ma to 300°C at 305 Ma. Such a 500°C temperature drop in 15 myrs represents a 33°C/myr cooling rate. Such a slow rate would require a constant heat flow during the Late Carboniferous in order to maintain a high mean crustal temperature.

The formation of the Velay migmatitic dome [START_REF] Ledru | The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution[END_REF]) is characterized by a Late Carboniferous continuous melting of the metasediments of the lower crust. Crustal melting, assisted by underplating of mantle-derived magma [START_REF] Williamson | Geochemical constraints on the genesis of Hercynian two-mica leucogranites from the Massif Central, France[END_REF]) from asthenospheric upwelling [START_REF] Ledru | The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution[END_REF], supports the existence of a regional high heat flow hypothesis in the Eastern Massif Central. However, this thermal anomaly remains limited to the Velay area, and indications of an important Westphalian and Stephanian magmatism are lacking in the Morvan, Vosges and Black Forest areas. The rare Stephanian shoshonites in the Epinac basin [START_REF] Chévremont | Notice explicative de la carte géologique d' Epinac-Les-Mines au 1:50000, Bureau de recherches géologiques et minières, Orléans Cocherie A, Albarède F[END_REF] cannot account for a continuous heat flow during the 315-300 Ma period. The most significant magmatic event recorded in Morvan, Vosges and Black Forest areas dates back to Early Permian times [START_REF] Lippolt | Isotopic evidence for the stratigraphic position of the Saar-Nahe Rotliegend volcanism, I 40Ar/40K and 40Ar/39Ar investigations[END_REF][START_REF] Campy | Le volcanisme acide de l'Ouest Morvan dans son cadre géologique: caractérisation géochimique, structurale et chronologique de mise en place[END_REF]. The current available data do not allow to determine which cooling path did the Autun and La Serre plutons follow. Whatever its path is, cooling was followed by ductile and brittle faulting along the Autun-La Serre transtensional fault system. The normal component deduced from field observations and microstructural analysis could have accommodated the rapid exhumation indicated by the cooling between 400°C and 300°C and by the rapid transition from ductile conditions along the shear zone at 300 Ma to brittle, normal, syn-sedimentary faulting at around 297 Ma (Figs. 8 and10a). Such a ductile-brittle continuum is known in the Aegean extensional realm [START_REF] Mehl | From ductile to brittle: evolution and localization of deformation below a crustal detachment (Tinos, Cyclades, Greece) Tectonics[END_REF] and also in the Massif Armoricain (Turrillot et al. submitted). In the Autun region, the ductile-brittle transition is also marked by a rotation of the stretching direction from NE-SW in Stephanian -Early Autunian times to N-S in Late Autunian times [START_REF] Marteau | Le bassin permo-carbonifère d'Autun. Stratigraphie, sédimentologie et aspects structuraux[END_REF][START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF]. Under brittle conditions, exhumation continued during Autunian times, accomodated by normal faulting. Evidence for Autunian normal faulting can be observed in the sedimentary basin of Autun, where at least 1200 m of detrital lacustrine and fluviatile sediments accumulated [START_REF] Chévremont | Notice explicative de la carte géologique d' Epinac-Les-Mines au 1:50000, Bureau de recherches géologiques et minières, Orléans Cocherie A, Albarède F[END_REF]. Moreover, in the sedimentary filling of the Autun basin, plant fragments were buried and transformed into coal.

Coalification requires rapid burial to ensure the preservation of the organic matter. The huge abundance of coal in the Autun basin suggests that the subsidence of the basin was tectonically controlled and was probably coeval with a rapid exhumation of the Variscan basement.

In the northeastern French Massif Central, the post-orogenic evolution is characterized by Namurian -Westphalian granite emplacement followed by Stephanian -Autunian NE-SW extension. This last stage contributed to the exhumation of the Variscan rocks, although it remains difficult to propose an exhumation rate for the Stephanian -Permian period since the geothermal gradient at that time is unknown and since the estimation of the P-T conditions of the granite before the initiation of the deformation is not available. The two-stages extension interpretation agrees with [START_REF] Faure | Late orogenic carboniferous extensions in the Variscan French Massif Central[END_REF] who pointed out that the Namurian -Westphalian extension is more pronounced in the western part of the French Massif Central than in the eastern part, where Stephanian to Permian extension is well recorded. The extensional processes thus appear to be subdivided into small intervals of regionally localized shearina and related to a large period of crustal melting.

Conclusion

New structural and geochronological data on the Autun and La Serre areas constrain the Late Variscan evolution of the northeastern part of the French Massif Central. The similarity between the Autun and La Serre shear zones suggests the existence of an ENE-WSW trending fault system, which was probably reactivated under brittle conditions during the Cenozoic. The major results include: 1) the recognition of normal-dextral ductile shear zones and 2) documentation of a diachronism of ca. 15 myrs between the Namurian -Westphalian pluton emplacement and the Stephanian -Permian extensional tectonics and basin subsidence. The Stephanian -Autunian Autun and La Serre shear zones affected the Namurian plutons during their post-emplacement cooling, but these plutons are not synkinematic. This shows that the widespread idea of synkinematic plutonism as as a characteristic of late-orogenic to post-orogenic extension suffers exceptions. Several points remain unclear, such as the tectonic setting during the emplacement of the Autun pluton.

More detailed studies, such as a study of the anisotropy of the magnetic susceptibility of the pluton, could improve our understanding of the Namurian -Westphalian extensional stage and its relationships with plutonism.

Figure captions

Figure 1: Geological map of the NW French Massif Central and the Vosges (modified after [START_REF] Fluck | Le socle vosgien, segment de la chaîne varisque d'Europe : projet Vosges : état des connaissances, propositions d'axes de recherches[END_REF][START_REF] Coromina | Late Palaeozoic NE-SW ductile-brittle extension in the La Serre horst, eastern France[END_REF][START_REF] Faure | Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain[END_REF]

NNW SSE SE03 v v v v v v v v v v v L A S E R R E S H E A R Z O N E L A S E R R E M E D IA N B R IT T L E F A U LT S Y S T E M L A S E R R E S O U T H E R N F A U L T v SE01 7 v v v a) c) b)
Figure 3 891

  . Location of the Autun and La Serre areas are also represented. AU: Autun Basin; EP: Epinac Basin; LC: Le Creusot Basin; SH: Sillon Houiller; MCSZ: La Marche-Combrailles Shear Zone; ASZ: Autun Shear Zone; SMF: La Serre Median Fault ; SMMF: Sainte-Marie-aux-Mines Fault.

Figure 2 :

 2 Figure 2: a): Simplifed geological map of the Autun area; b) cross section of the Autun

Figure 3

 3 Figure 3: a): Simplifed geological map of the la Serre area (modified after Coromina

Figure 4 :

 4 Figure 4: Microstructure of the granite and the mylonite of the Autun and La Serre

Figure 5 :

 5 Figure 5: 3D fabric of Autun granites and mylonites deduced from preferred

Figure 6 :

 6 Figure 6: Universal Stage measurements of the lattice preferred orientation of the

Figure 7

 7 Figure 7: a: Photomicrograph of a monazite grain enclosed in biotite in the La Serre

Figure 8 :

 8 Figure 8: 40 Ar/ 39 Ar age spectra of Autun granite and mylonite (a to e)) and La Serre

Figure 9 :

 9 Figure 9: Simplified basement map of northeastern France. Deep Carboniferous basins

Figure 10 :

 10 Figure 10: Synoptic chart of new geochronological data, showing that a ca. 15 myrs

Fig. 3

 3 Fig.3

  Fig.2

  Figure 4 893

Table 1 40

 1 Ar/ 39 Ar analytic data.

Acknowledgements

This work is a part of F. Choulet's master thesis dissertation. Field and analytical expenses have been funded by ISTO and Chrono-environnement. Alain Cocherie is acknowledged for his help during monazite U-Th-Pb calculations. The authors thank G. Eisbaher and J-B Edel, for constructive review.