The role of the Ibero-Armorican arc in the tectonic inversion of the Sout-Portuguese Zone (SW Iberian Massif, Variscan Orogeny)

Abstract

The SW Iberian Massif tells an important part of the history of the late Paleozoic tectonic evolution of northern Gondwana margin, and specially that of the Rheic Ocean. The closure of the Rheic Ocean and consequent continental collision between Gondwana and Laurussia which ultimately led to the amalgamation of Pangea, gave rise to the European Variscan belt (Matte, 2001). The Variscan belt is linear but sinuous with kilometre-scale curvatures.

In Western Europe, the Ibero-Armorican Arc (Ribeiro et al., 1995) includes a fold and thrust belt affecting a foredeep basin in the outer arc (South Portuguese Zone- SPZ), and a fold and thrust belt affecting shallow water continental platform in the inner arc (Cantabrian Zone). These foreland basins are separated by an hinterland (Ossa-Morena Zone, Central-Iberian Zone and West Asturio-Leonese Zone) dominated by Mississipian/early Pennsylvanian high-grade to low-grade metamorphism, synorogenic plutonism and sedimentation, and heterogeneous deformation (Variscan orogeny).

The originally linear Variscan belt was the result of (1) an oblique-subduction process that culminated in the late Devonian (c. 370 Ma) and consequent crustal thickening with subsequent orogenic collapse during the Mississipian/early Pennsylvanian (Tournaisian to early Moscovian; c. 350-310 Ma) (Martinez-Catalan et al., 2007). The orocline formation occurred as a secondary structure in the middle-late Pennsylvanian (late Moscovian to late Gzhelian; c. 306-299 Ma) and was followed by significant late-post-Variscan magmatism (early Permian; c. 295 Ma) (Weill et al., 2010 and references therein).

The geology of SPZ, located in the outer arc, includes a Carboniferous foreland basin and a late Devonian basement characterized by shallow water siliciclastic sediments (Horta da Torre Formation, Represa Formation, PQ Formation and Tercenas Formation) (Oliveira et al., 1990). The Famennian rocks are overlained: (1) by a succession with Tournaisian-Visean felsic volcanism (Volcano-Sedimentary Complex), and late Visean (Mertola Formation) to Serpukovian-Bashkirian (Mira Formation) overlying turbidites, or (2) by a succession of Tournaisian (Bordalete Formation), Visean (Murração Formation) and middle Bashkirian age (Quebradas Formation) mud/carbonate rocks. Lower Bashkirian to late Moscovian (Brejeira Formation) turbidites commonlly rest conformably on Serpukovian-Bashkirian sediments but also unconformably overlie sediments with Tournaisian to Visean ages (Pereira et al., 1994).

As response to middle-late Pennsylvanian tectonic inversion, the overall structure of the SPZ is characterized by southwestward (present coordinates) vergent folding and thrust displacement (Silva et al., 1990). This complex contraction deformation obscured the importance of the Viséan syn-sedimentary gravitational collapse structures and extensional faults. In Mértola we recognized tectonic instability during the late Visean with deposition of thick turbiditic sequences coeval with large-scale slope mass wasting of the older platform margin. The Visean to Moscovian stratigraphy of Aljezur-Bordeira reveals significant lateral changes of sedimentary facies and the presence of unconformities suggesting that sedimentation was influenced by development of synsedimentary extensional faults. The regional folding and thrusting that characterizes the overall structure of the SPZ, with the same age of the Ibero-Armorican Arc development, clearly marks a tectonic inversion that postdated late Visean to late Moscovian deposition contemporaneous with the extensional collapse and significant up-lift and denudation of the Variscan linear orogen (Pereira et al., 2009).