Pre-Variscan tectonics in Sardinia: insight into Lower Palaeozoic geodynamic processes along the Gondwanan margin

The tectonic evolution of continental palaeo-margins involved in the Variscan Orogeny remain debated. Along the northern Gondwana margin, contrasting Ordovician geodynamic settings have been proposed, including rifting, ocean spreading, passive margins, subduction–accretion complexes, non-collisional orogens, and volcanic arcs. Southern Sardinia represents a key area to investigate these processes, owing to the very low-grade Variscan metamorphic overprint and the limited post-Variscan deformation.

Here we reconstruct the Ordovician tectonic evolution of the Variscan basement of southern Sardinia through the analysis of stratigraphic architectures and structural features of the External and Nappe zones, which record two distinct but partly coeval geodynamic scenarios.

The External Zone is characterized by two main stratigraphic successions separated by the regional Sardic angular unconformity. The lower succession (Cambrian–Lower Ordovician) comprises a basal terrigenous unit with minor limestone intercalations, overlain by a thick carbonate platform and upper siliciclastic deposits. The overlying Upper Ordovician succession starts with coarse conglomerates that grade upward into finer-grained siliciclastic deposits.

The Nappe Zone consists of three stratigraphic successions separated by the Sarrabese angular unconformity and the Katian nonconformity. These include: (i) a Cambrian–Lower Ordovician terrigenous succession with interlayered volcanic levels; (ii) a Middle–Upper Ordovician volcano-sedimentary succession; and (iii) an Upper Ordovician succession dominated by siliciclastic deposits. In both zones, Silurian–Devonian black shales and limestones are overlain by syn-orogenic Lower Carboniferous deposits.

The Sardic and Sarrabese unconformities are interpreted as the result of folding events (Sardic and Sarrabese tectonic phases) affecting the Cambrian–Lower Ordovician successions. Their precise ages remain poorly constrained and are likely not synchronous, as suggested by the different durations of the associated stratigraphic gaps (ca. 17 Ma for the Sardic unconformity and ca. 6 Ma for the Sarrabese unconformity).

The post-Sardic stratigraphic evolution of the External Zone is consistent with non-volcanic rifting, which initiated approximately 10 Ma after the onset of subduction-related volcanic arc activity recorded in the Nappe Zone. These contrasting geodynamic settings coexisted for at least ~8 Ma during the Sandbian to early Katian. During this interval, the External Zone evolved along a divergent margin, whereas the Nappe Zone was part of a convergent margin characterized by active arc magmatism. Volcanic activity ceased during the middle Katian, marking the transition to passive margin conditions above the former arc.

The coexistence of contrasting tectonic evolutions in coeval stratigraphic successions suggests that the External and Nappe zones occupied distinct palaeogeographic positions along the same continental margin, likely separated by large distances along the northern Gondwana margin, without evidence for intervening oceanic basin closure. During the Early Carboniferous, Variscan tectonics ultimately assembled these domains into their present configuration, with the Nappe Zone thrust above the External Zone.