Tectono-sedimentary evolution of the Late Palaeozoic-Early Mesozoic metasediments of the Monticiano-Roccastrada Ridge (southern Tuscany, Northern Apennines, Italy)

In the Northern Apennines, rocks of Late Carboniferous to Carnian age unconformably overlie pre-late Moscovian successions which were deformed and locally metamorphosed during the final consolidation of the Variscan orogen. The latest Carboniferous to Carnian successions were affected by Alpine tectono-metamorphic events and are now preserved in the deepest tectonic units of the Northern Apennines (Tuscan Metamorphic Units). In Southern Tuscany, they are mostly exposed along the morpho-structural alignment referred to as the Monticiano-Roccastrada Ridge (MRR). Despite the Alpine tectonic and metamorphic overprints, the sedimentary features of these rocks are still recognisable and suggest that they represent the infill of a variety of continental to marine, long- to short-lived basins located in the Tuscan palaeogeographic portion of Gondwanaland. The review of the literature, improved with new stratigraphic data on the Late Palaeozoic units cropping out along the MRR, led to provide a palaeogeographic and stratigraphic scenario more complex than previously known in southern Tuscany. Specifically, five informal unconformity-bounded stratigraphic units (UBSU), developed in the Middle Carboniferous-Middle Triassic p.p. time span, and a Middle p.p.-Late Triassic Verrucano s.s. succession, have been recognised in the Late Palaeozoic- Early Mesozoic metasediments of the MMR. The recognised UBSU are, from oldest: 1) synthem A, represented by the Moscovian Sant’Antonio Limestone; 2) synthem B, comprising the late Moscovian-Kasimovian? Spirifer-bearing Schists; 3) synthem C, which is Middle-Late Permian-earliest Triassic? in age and includes the Farma, Poggio al Carpino, Carpineta and Poggio alle Pigne fms.; 4) synthem D, consisting of the latest Permian?- earliest Triassic? Fosso Pianaccia Conglomerate; and 5) synthem E, comprising the Early?-Middle Triassic p.p. Civitella Marittima Fm. The unconformably overlying Verrucano s.s., comprising the Monte Quoio and Anageniti Minute fms., manifests the base of the Alpine sedimentary cycle, even though it cannot be considered a synthem since it is not topped by an unconformity. Except for synthem D, interpreted as the continental infill of erosional depression, stratigraphic-sedimentological evidence along with the regional geodynamic framework, all suggest that extensional/transtensional tectonics was the main controlling factor, producing sedimentary basins where continental to marine successions deposited unconformably over pre-Late Carboniferous rocks and/or earlier successions. On the whole, before the onset of the main Triassic rift phase (recorded by the Verrucano s.s.), five main tectono-sedimentary events are recorded in the MRR.