Oroclinal arcs of the Variscan Belt: features and mechanisms

Arcuate trace of large structures characterizes many mountain chains. The Variscan Belt is not an exception, and depicts one of the tightest oroclines in the world, the Ibero-Armorican Arc. In addition, the belt features a few more open arcs in the Eastern Moroccan Meseta, Central Iberia, the French Massif Central and the Bohemian Massif. All Variscan arcs are considered oroclines or secondary oroclines according to definitions by Weil and Sussman (2004) and Johnston et al. (2013) respectively. They are also essentially late orogenic features, but their timing and deformation mechanisms differ. Models explaining their origin have been proposed for some individual arcs, and are generally controversial.

This contribution aims at interpreting the ensemble of Variscan arcs paying attention to their age relative to previous orogenic features as well as to those associated with arc development. Such features include first order structures, metamorphism and plutonism, as well as magnetic and gravimetric anomalies. Development of the arcs is viewed as somehow related with late Variscan dextral transpression provoked by the relative displacement of Laurussia to the East relatively to Gondwana during the Pennsylvanian and early Permian (325-290 Ma; Arthaud and Matte, 1977; Shelley and Bossière, 2000; Martínez Catalán et al., 2021). But several mechanisms operated to form the arcs, the most important of them being ductile transcurrent shearing, indentation and shortening perpendicular and parallel to the orogenic trend. These mechanisms acted at different time intervals and their participation or relative importance varies for each arc, as well as their involvement in the development of the structural, metamorphic and igneous features and in the geophysical characteristics.

REFERENCES:

Arthaud, F. and Matte, P. 1977. Late Paleozoic strike-slip faulting in southern Europe and northern Africa: result of a right-lateral shear zone between the Appalachians and the Urals. Geological Society of America Bulletin, 88, 1305-1320.

Johnston, S.T., Weil, A.B. and Gutiérrez-Alonso, G. 2013. Oroclines: Thick and thin. Geological Society of America Bulletin, 125 (5-6), 643-663.

Martínez Catalán, J.R., Schulmann, K. and Ghienne, J.F. 2021. The Mid-Variscan Allochthon: Keys from correlation, partial retrodeformation and plate-tectonic reconstruction to unlock the geometry of a non-cylindrical belt. Earth-Science Reviews, 220, 103700, 1-65.

Shelley, D. and Bossière, G. 2000. A new model for the Hercynian Orogen of Gondwanan France and Iberia. Journal of Structural Geology, 22 (6), 757-776.

Acknowledgement: Spanish Ministry of Science and Innovation, project PID2020-117332GB-C21.