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

Abstract : 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 high-temperature 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 40Ar-39Ar datings of the Autun granite yield plateau ages from 305 to 300 Ma. The ca. 300 Ma 40Ar-39Ar 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 postorogenic 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.
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Flavien Choulet, 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. International Journal of Earth Sciences, Springer Verlag, 2012, 101 (2), pp.393-413. ⟨10.1007/s00531-011-0673-z⟩. ⟨insu-00596918⟩

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