Ordovician geodynamics of the Sardinia block: a key for the reconstruction of the pre-Variscan paleogeography

The crystalline basement of the Sardinia block is made up of an almost complete segment of the Variscan belt. Along a SW-NE transect of roughly 200 km, it is possible to observe the structure of the chain from the shallowest to the deepest domains, starting from an anchimetamorphic external zone in the SW of the Island, to a green-schist facies nappe zone in the center and to a medium to high-grade metamorphic inner zone in northern Sardinia. The exceptional exposure of the chain in Sardinia makes it an essential piece for the reconstruction of the pre-Variscan geodynamics and the Paleozoic terranes puzzle.

In several reconstruction of the pre-Variscan paleogeography, Sardinia is considered a whole single block that experienced, since Cambrian times, several geodynamic settings as part of the northern Gondwana margin, before being involved in the Variscan Orogeny during lower Carboniferous. As stated by previous Authors, the most relevant pre-Variscan geodynamic events recorded in Sardinia occurred during the Lower-Middle Ordovician, when the Sardinian block was located close to a subduction zone where a volcanic arc developed. According to this interpretation, the external, nappe and inner zones acted as back-arc, arc and fore-arc, respectively, belonging to the same lithospheric block.

The main evidences of Ordovician tectonics and volcanic activity are a folding event that affect only the Cambrian-Lower Ordovician successions and an angular unconformity related to the folds sealed by continental and tidal deposits in the external zone and by calc-alkaline volcanic products in the nappe zone.

The review of the paleontological, stratigraphic, magmatic and structural data highlights significant discrepancies between the external and nappe zones, suggesting that these domains did not share the same geodynamic setting and, possibly, paleogeographic position during the Ordovician, implying they drew close and amalgamated only in Variscan times.

This hypothesis is supported by the different ages of the unconformities, Upper Ordovician in the nappe zone and Middle Ordovician in the external zone, and the extent of the stratigraphic gap, long-lasting in the external zone. Furthermore, the activity of the volcanic arc in the nappe zone is contemporaneous to the continentalization and erosive processes in the external zone, that is totally devoid of magmatism and volcano-sedimentary deposits. The Upper Ordovician succession in the external zone define 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. Note that also the faunas show remarkable differences between the external and nappe zones. Finally, a different paleogeographic position is suggested by the Hirnantian glaciomarine deposit in the external zone, lacking in the nappe zone.

The recognition that the Sardinian block consisted of two distinct terranes before the Variscan Orogeny, entails alternative correlations and an adjustment of the arrangement of the now scattered Variscan terranes. In particular, the external and nappe zones should be located in different positions in order to fit the proper geodynamic setting in the paleogeographic reconstruction at the snapshot time.