21 de January de 2022
Casas, J. M., Guimerà, J., Alvarez-Marron, J., & Días da Silva, Í. (2022). Is the Ibero-Armorican Arc primary or secondary? An analysis of the contraction required to form it by vertical axis rotation. Journal of the Geological Society, jgs2021-065. https://doi.org/10.1144/jgs2021-065
Abstract
Different models have been proposed to explain the formation of the Ibero-Armorican Arc (IAA), as being primary or secondary (rotational). Secondary arc formation models require significant vertical axis rotations at the end of the Variscan orogeny. Estimates of the north-south (present coordinates) amount of contraction (horizontal shortening) needed for these rotations range from 54% to 91%, at the core or at the SE tip of the arc, near the present-day Mediterranean coast, respectively. These estimates are compared with coeval deformational structures developed in three areas of the orogen, one in the autochthonous hinterland in the southern branch of the arc, the second in the Cantabrian Zone foreland fold-and-thrust belt in the core of the arc, and the third in the SE continuation of the south branch of the Ibero- Armorican Arc, in the present-day Iberian Chain. From this analysis it follows that the shortening associated with late Variscan deformation is much less than that needed to explain the formation of the Ibero-Armorican Arc from a former linear orogen by means of vertical axis rotation. We therefore propose that the geometry of the arc should be re-evaluated. We discuss the paleomagnetic data purported to support the secondary origin, as well as the role of late-Variscan regional strike-slip faults in modifying the geometry of the Iberian and Armorican massifs. Our analysis suggests that the IAA formed mainly from a primary non-rotational margin-controlled curvature, slightly modified by ca. 5-20% of superposed contraction during late Carboniferous and/or Alpine times.