The main volcanic rhyolitic event in NW Sardinia (Nurra) and Permo-Triassic successions outcrop between Punta Capparoni and Cala Viola, where they attain the maximum thickness (>300 m), northwards undergo sudden reduction. In particular at Mt. Santa Giusta, the "Autunian" succession was not deposited, and the thickness of the Permo-Triassic red beds decreases to few metres above the ignimbrite that rests directly on the basement. New <sup>50</sup> Ar- <sup>39</sup> Ar step heating technique on biotite separates gives 291.5+ or -1.5 Ma for this event; this age is considerably older than existing radiometric data. The corresponding Lower Sakmarian age and the field relationships with the Permo-Triassic successions Nurra to the S, allow to assess that north of Porto Ferro the Nurra basement was a structural high bounded by E-W trending faults since the Late Carboniferous-Lower Permian, and that the same faults that controlled the post-collisional Variscan extension Also controlled the development of Mid Permian and Lower Triassic successions.

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