Paleogeographic inversion resulting from large out of sequence breaching thrusts: The León Fault (Cantabrian Zone, NW Iberia). A new picture of the external Variscan Thrust Belt in the Ibero-Armorican Arc

Abstract

Out of sequence breaching thrusts may give rise to duplication of a former thrust stack in map view in the same manner that stratigraphic units are repeated by initial thrusts. In this way, large breaching thrusts may put an inicial paleogeographic pattern out of order, producing apparent paleogeographic inversions at regional scale. The León Fault, an orogen-scale fault located in the Variscan foreland fold-thrust belt of the Iberian Peninsula, known as the Cantabrian Zone, has caused much controversy in the understanding of Iberian Paleozoic tectonics. The León Fault displays all the features of a breaching thrust and from this interpretation a simple paleogeographic pattern results, once both breaching and former thrusts are restored, in contrasts with previously reported paleogeographic models. This approach implies a redefinition of the geological provinces of the Cantabrian Zone, since some major nappes of that zone, so far considered as different tectonic units, are reinterpreted as one single unit repeated by the León Thrust. It also has implications in discriminating between the various kinematic models proposed for the larger Ibero-Armorican Arc, favouring a late bending of an initial linear belt instead of a progressive change in transport direction during nappe emplacement. Finally, the paleogeographic inversion caused by the León breaching Thrust is compared to other orogen-scale paleogeographic inversions associated with antiformal stacks, such as the preAlps with respect to the Helvetic Nappes, and the Ligurian-Tuscan domains in the Apennines.