Wrench-Related Dome Formation and Subsequent Orogenic Syntax Bending in a Hot Orogen (Variscan Ibero-Armorican Arc, the Ouessant Island, France)
Résumé
During the Carboniferous collision stage, the West European Variscan orogen was affected by oblique convergence, wrenching, plate-scale oroclinal bending, and widespread exhumation of the deep crust. One of these exhumed units forms the Léon dome located on the northern flank of the Ibero-Armorican Arc in the western part of the Armorican massif. Structural field data from the Ouessant Island reveal kinematic changes between 330 and 300 Ma that affected the northwestern margin of the Léon dome. This margin underwent two ductile deformation phases with opposite strike-slip shear senses. Dextral strike-slip wrenching combined with orogen-normal shortening, orogen-parallel stretching, and local top-to-the-NE shearing occurred during the Léon dome formation and exhumation of its gneissic core at 330-310 Ma. During tightening of the Ibero-Armorican Arc at 300 Ma, more localized sinistral wrenching reactivated the northwestern boundary of the Léon dome. The resulting N70°E trending sinistral Porspoder-Ouessant shear zone together with its conjugate dextral South Armorican shear zone controlled the lateral escape of the Léon-North-Central Armorican rigid tectonic wedge triggered by indentation of the Cantabrian orocline at the core of the Ibero-Armorican Arc.
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