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Abstract 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. 

Abstract In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension‐driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high‐pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well‐preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites recordP ~ 1.5 ± 0.2 GPa atT ~ 700 ± 20°C, but differ from each other in whole‐rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss atc.310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome‐margin eclogite zircon retains a widespread record of protolith age (c.470–450 Ma, the same as host gneiss protolith age), whereas dome‐core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust atc.310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low‐P/high‐Tconditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome. 

Continents are constantly moving, and sometimes they collide. When continents collide, they crumple, and thicken. Mountain ranges form in this “crash zone.” Deep rocks at the bottom of a crash zone are hot because they are so deep. Hot materials—even rocks—become weak. Hot rocks deep underground can move by flowing, even though they are mostly solid. First, they flow sideways and then upwards in large blobs. When upward-moving blobs are only a few kilometers below the surface of the Earth, they cool and harden into bell shapes (domes). Flowing rocks cause the crash zone to collapse and spread out. Continents go back to their pre-collision thickness. They are not exactly the same as before collision, though: some rocks that used to be at the bottom of the continents are now at the top! We can see these formerly deep parts of continents in rock domes all over the world.