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 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 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.

Sedimentology and U-Pb dating of Carboniferous to Permian continental series of the northern Massif Central (France): Local palaeogeographic evolution and larger scale correlations

“…This extensive event is often attributed to the post-orogenic collapse of a thickened and hot crust (Ménard and Molnar, 1988;Echtler and Malavieille, 1990). In this geodynamic framework, late Carboniferous-Permian sedimentation is thought to occur in intramontane coal-basins, usually interpreted as half-grabens associated with major detachment faults (Malavieille et al, 1990;Van Den Driessche and Brun, 1991;Malavieille, 1993;Faure, 1995), or as pull-apart basins by reactivation of major shear zones (Vallé et al, 1988;Genna et al, 1998;Choulet et al, 2012).…”

Chronology, petrogenesis and heat sources for successive Carboniferous magmatic events in the Southern–Central Variscan Vosges Mts (NE France)

“…On the other hand, the Central Vosges Granite magmatism can be correlated with a major tectonomagmatic event associated with extensional tectonics affecting much of the Massif Central further west (e.g. Faure et al 2009a,b;Choulet et al 2012). This is generally attributed to a gravitational collapse of the thickened crust, which, in theory, could be accommodated by extension characterized by thermal relaxation and partial melting of the continental crust (England & Thompson 1986;Vanderhaeghe & Teyssier 2001a).…”

Strain Partitioning Within Bending Orogens, New Insights From the Variscan Belt (Chiroulet‐Lesponne Domes, Pyrenees)

“…It is associated with high-to ultra-high-temperature metamorphism (Barbey et al, 2015;Kunz et al, 2018;Pin & Vielzeuf, 1983) and bimodal magmatism that has affected the whole Franke (2014) for the northern realm of the Variscan belt and Stampfli et al (2013) for the southern realm (Alleghanian belt, Alpine Mediterranean domain, and Paleotethys) with modifications for Corsica, Sardinia, and Maures-Tanneron relative positions by considering recent paleomagnetic syntheses by Edel et al (2014Edel et al ( , 2018. Directions of stretching are represented only for the 310-290 Ma period and were synthesized from (1) Michard et al (2010), (2) review from Martínez Catalán et al ( 2014), (3) Cochelin et al (2017), (4) Corsini and Rolland (2009), (5) Carosi and Palmeri (2002), (6) Casini et al (2012), (7) Choulet et al (2012), (8) Ledru et al (2001), (9) Echtler and Malavieille (1990), (10) Faure et al (1990), (11) Faure et al (2009), (12) Genna et al (1998) and Rolin and Colchen (2001), (13) review from Gapais et al (2015), ( 14) Saspiturry et al (2019), (15) Carreras and Debat (1996), ( 16) de Saint Blanquat (1993), (17) Díez Fernández et al (2012, (18) Escuder Viruete et al (19) Hernández Enrile (1991). Abbreviations: CZ, Cantabrian Zone; CIZ, Central Iberian Zone; FMC, French Massif Central; AM, Armorican Massif; BM, Bohemian Massif; MN, Montagne Noire; MT, Maures-Tanneron; CS, Corsica-Sardinia.…”