Geological Setting
Although heterogeneously deformed by the Late Paleozoic Variscan Orogeny, northwestern Iberia exposes one of the most complete sections of the Paleozoic northern Gondwana passive margin. In this region, Paleozoic rocks lie within the tightly curved Cantabrian Arc. If the arc is restored to a pre-Variscan geometry (Weil et al., 2001; Weil et al., 2010; Gutiérrez-Alonso et al., 2012), the Iberian portion of the Gondwana continental platform is shown to be extensive and positions northwest Iberia adjacent to West Africa along the southern flank of the Rheic Ocean throughout the Paleozoic (Robardet, 2002; Robardet, 2003; Martínez Catalán et al., 2007; Nance et al., 2010).
The Paleozoic rocks of the Iberian Massif are traditionally divided into zones based on differences in their Lower Paleozoic sedimentary successions, which are interpreted to reflect their relative proximity to the Gondwanan margin. From the ancient coastline seaward towards the Gondwanan outer platform, five zones are identified. The Cantabrian Zone (CZ) constitutes the foreland fold and thrust belt. The CZ consists of a sedimentary sequence of pre-orogenic, lower Paleozoic platform sedimentary rocks that thin towards the core of the arc. These pre-orogenic strata are covered by a Carboniferous syn-orogenic sequence and a post-orogenic (Stephanian) continental succession (Pastor-Galán et al., 2011). Structurally, the CZ is characterized by thin-skinned tectonics with a transport direction towards the core of the arc (Pérez-Estaún et al., 1988). The basal thrust for the whole imbricate system is imaged as a flat surface in a deep seismic survey parallel to the axial trace of the Cantabrian Arc (Pérez-Estún et al., 1994). Given the lack of internal strain and metamorphism, and the presence of a complete syn-orogenic sedimentary sequence in the CZ, it is an ideal zone for combined paleomagnetic and progressive deformation structural studies. The West Asturian Leonese (WALZ), Central Iberian (CIZ) and Galicia Tras-os-Montes (Schistose Domain) Zones, together with the Ossa-Morena Zone of southern Iberia, preserve a more outboard tectonostratigraphy (Julivert et al., 1972; Quesada, 1990; Ribeiro et al., 1990; Perez Estaun et al., 1991; Quesada, 1991; Martinez Catalan et al., 1997; Gutiérrez-Marco, 1999; Marcos and Farias, 1999; Martínez Catalán et al., 1999; Aramburu, 2002; Robardet, 2002; Robardet, 2003; Robardet and Gutiérrez-Marco, 2004) and constitute the orogenic hinterland. Boundaries between these zones are major Variscan thrust faults that were, in some cases, reactivated by extension in the aftermath of the Variscan Orogeny (Martinez Catalan et al., 1997; Martinez Catalan et al., 2003). Closure of the Rheic Ocean is recorded in northwest Iberia by deformation attributable to the Laurussia–Gondwana collision, referred to as the Variscan Orogeny, and by the emplacement of ophiolite interpreted as oceanic remnants thrust out of the Rheic suture between these continents.
The onset of continental collision began at ca. 365 Ma (Dallmeyer et al., 1997) with initial subduction of the Gondwana margin below Laurussia. Deformation of the Gondwana passive margin succession migrated eastward in space and time and was caused by Laurussia overriding Gondwana. Convergence initially produced recumbent folds (D1) that verge and migrate from the suture towards the Cantabrian Arc. Continued shortening is thought to have led to the extensional collapse (D2) of the thickened orogenic hinterland (Viruete et al., 1994; Arenas and Catalan, 2003; Pereira et al., 2009) at ca. 320 Ma (Martinez Catalan et al., 2009). Extensional collapse within the hinterland was coeval with the development of a non-metamorphic foreland fold-thrust belt within the Cantabrian Zone (Perez Estaun et al., 1994). Immediately following ocean closure, an abrupt change in the stress field associated with Pangea amalgamation caused a dramatic 180° rotation of the Variscan Belt to produce the Ibero-Armorican Orocline.