The Ordovician Sardic and Sarrabese phases in the South European Variscan Belt: correlation between Sardinia, Eastern Pyrenees and Occitan Domain

Cocco Fabrizio;Loi Alfredo;Funedda Antonio Luca;Casini Leonardo;Ghienne Jean Francois;Pillola Gian Luigi;Vidal Muriel;Meloni Mattia Alessio;Oggiano Giacomo

2022-01-01

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Abstract

Evidence of Ordovician tectonics is well preserved in the Variscan terrains now scattered in the circum-Mediterranean areas. It was in SW Sardinia (the External Zone of the Variscan basement) that was firstly detected the so-called Sardic Phase, a folding event that affected only the Cambrian-Lower Ordovician succession, and the related angular unconformity (Sardic unconformity). At a later time, a folding event (Sarrabese Phase) affecting a Cambrian-Lower Ordovician succession and an angular unconformity (Sarrabese unconformity) have been detected also in SE Sardinia, in the shallowest tectonic unit (Sarrabus Unit) of the Variscan Nappe Zone, and the Sardic and Sarrabese phases (and their unconformities) were considered the same event. However, comparing the Cambro-Ordovician succession, the tectonic structures and the bio-stratigraphic data, relevant discrepancies arise between the External and Nappe Zone, suggesting that these domains did not share the same geodynamic setting and paleogeography in pre-Variscan times. Noteworthy are the different extent of the unconformity-related gaps (17 and 6 Ma in the External and Nappe zones, respectively), a lower Cambrian thick limestone formation outcropping only in the External Zone, and the occurrence only in the Nappe Zone of a subduction-related magmatic arc that seals the Sarrabese unconformity. Note that the activity of the volcanic arc in the Nappe Zone is contemporaneous to the continentalization and erosive processes in the External Zone. Furthermore, the Upper Ordovician succession in the External Zone defines a rift that evolve to a passive margin, whereas in the Nappe Zone the onset of a passive margin is marked by a nonconformity above the volcanic arc. This evidence supports the idea that the Sardinian block consisted of two distinct, not contiguous terranes before the Variscan Orogeny, entailing alternative correlations and an adjustment of the arrangement of the now scattered Variscan terranes. The Eastern Pyrenees and Occitan domains show Ordovician features similar to that of Sardinia, and up to now, in several paleogeographic reconstructions, Sardinia, Pyrenees and Occitan domains have close positions. However, comparing the stratigraphic and tectonic features in view of the evidence exposed above for the Ordovician of Sardinia, we infer that also in the Paleozoic basement of the Pyrenees and Occitan domain two distinct pre-Variscan terrains can be recognized. This suggests a correlation either with the External or the Nappe Zone and an involvement in two different geodynamic settings, according to the Sardic or Sarrabese tectonic evolution styles. To conclude, the data from the Ordovician of Sardinia could be an interpretation key for a more accurate reconstruction of the early Paleozoic paleogeography of Gondwana as well as the complex dynamics that led to the Variscan Orogeny.