A Reappraisal of Nappe Structures and Strain Patterns in the Core of the Variscan Orogen (French Massif Central)

Major influences on the architecture of orogens include the overall thermal conditions of orogeny (hot vs. cold) and the angle of collision (orthogonal vs. oblique). In the French Massif Central of the Variscan orogen, a cold-orogen style crustal nappe architecture was interpreted in the Haut-Allier, located in the core of the orogen. Based on this model, the Variscan orogenic crust is described as the superposition of three main allochthonous tectonic units juxtaposed along major thrust faults. However, based on a detailed structural analysis, we have found instead that the region is dominated by a network of anastomosing conjugate and coeval dextral and sinistral shear zones striking WNW-ESE and ENE-WSW, respectively. The dextral WNW-trending shear zones are predominant, whereas the sinistral NE shear zones are mainly located in the eastern part of the massif. Between these sub-vertical shear zones, a sub-horizontal foliation is observed. Our results indicate that all planar fabrics were partially synchronous during suprasolidus low-pressure-high-temperature conditions. Strain partitioning occurred from high-temperature suprasolidus conditions to subsolidus retrogression and may represent orogen-parallel flow, suggesting hot-orogen style. These results call into question the validity of the crustal nappe model in the Haut-Allier. Based on new structural data and related observations, we propose a new model in which metamorphic gaps between tectonic units are explained by the juxtaposition of different structural domains by displacement along strike-slip shear zones.