Communication Dans Un Congrès Année : 2025

Deconvolution of the finite strainfield from the western French Massif Central: implications on the exhumation of HP rocks and Variscan geodynamics

Résumé

The French Massif Central (FMC) forms the hinterland of the west-European Variscan belt and underwent numerous tectonic and metamorphic events from Devonian to Permian periods. The accreted tectonic units belonging to Gondwana and Laurussia were involved in subduction, nappe stacking and syn-to post-orogenic extension to varying degrees. This orogenic evolution suggests that the resulting finite structure likely reflects the interference between fabrics formed at different stages of the orogenic cycle with varying degrees of overprinting, depending on different factors such as spatial strain gradients, lithologies, structural levels or the degree of partial melting. We present a model of finite strain field of the western FMC, based on new structural data from the Limousin area and an extensive compilation of available data. Lineations and foliations were analyzed and interpolated at regional to local scales, using geostatistics. Ductile fabrics within autochthonous units and syn-collisional plutons are poorly disturbed and reflect the transition from syn-thickening to extensional lateral flow of the partially molten mid-lower crust. In contrast, allochthonous units are affected by numerous perturbations, including polyphased ductile shearing, folding and faulting, which induce local block rotations. Considering the overprint of late tectonic events, we show that early fabrics related to high pressure metamorphism or nappe stacking (D1, D2) have undergone significant reorientation. As a result, the finite trend of the associated lineations (L1-L2) must be treated with caution and may not always reflect the original direction of stretching and shearing that occurred during the early compressional stages. By re-analyzing the Lauriéras and Argentat seismic profiles (Limousin), we propose that the allochthonous units are affected by block-tilting induced by listric normal faults that merge at depth, forming the top of an extensional gneiss dome. Consequently, ductile fabrics within allochthonous units are early and should not be interpreted as related to NW-SE syn to post-orogenic extension, as recently proposed for the eastern FMC. Considering i) the spatial distribution and various degrees of reorientation of these early fabrics, and ii) the deconvolution of these disturbances, we propose the following: -The high dispersion of lineation trends in allochthonous units of Limousin can be explained by passive rotations, during later isoclinal folding under dextral transpressional conditions, rather than by two superposed tectonic phases (the so-called D1& D2), as previously inferred, -The allochthonous units from the Limousin form a single tectonic unit, -This main event of ductile deformation in the allochthonous units may be related to the exhumation of the subducted continental crust rather than nappe stacking, -The formation of Variscan orogenic prism in the western FMC may only result of continuous south-directed thrusting rather than a switch from NE-SW to NW-SE thrusting.

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Dates et versions

hal-05098681 , version 1 (05-06-2025)

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  • HAL Id : hal-05098681 , version 1

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Bryan Cochelin, Charles Gumiaux, Florence Cagnard, Adnand Bitri. Deconvolution of the finite strainfield from the western French Massif Central: implications on the exhumation of HP rocks and Variscan geodynamics. geotolosa2025, Jun 2025, Toulouse, France. ⟨hal-05098681⟩
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