Reconsidering the Variscan basement of southern Tuscany (inner Northern Apennines)

Abstract

<p>The Pre-Mesozoic units exposed in the inner Northern Apennines mostly consist of middle-late Carboniferous-Permian successions unconformably deposited on a continental crust consolidated at the end of the Variscan (i.e. Hercynian) orogenic cycle (Silurian-Carboniferous). In the inner Northern Apennines, exposures of this continental crust, Cambrian?-early Carboniferous in age, have been described in the Northern Tuscany, Elba Island (Tuscan Archipelago) and, partly, in scattered and isolated outcrops of southern Tuscany. In this contribution, we reappraise the most significative succession (i.e. Risanguigno Formation) exposed in southern Tuscany and considered by most authors as part of the Variscan Basement. New stratigraphic and structural studies, coupled with palynological analyses, allow us to refine the age of the Risanguigno Fm and its geological setting and evolution. Based on the microfloristic content, the structural setting and the fieldwork study, we attribute this formation to late Tournaisian-Visean (middle Mississipian) time interval and conclude it is not showing evidence of a pre-Alpine deformation. These results, together with the already existing data, allow us to presume that no exposures of rocks involved in the Variscan orogenesis occur in southern Tuscany.</p>

Cyclic Brittle‐Ductile Oscillations Recorded in Exhumed High‐Pressure Continental Units: A Record of Deep Episodic Tremor and Slow Slip Events in the Northern Apennines

“…N‐S trending thrusts into the Iano‐, the Monte Quoio‐Montagnola Senese‐ and the Monte Leoni‐Farma subunits (form structurally higher to lower and from internal to external; Figure 1, Costantini et al., 1988; Lazzarotto et al., 2003). The Monte Quoio‐Montagnola Senese subunit is well exposed along the Farma River, where it appears to be composed of three metasedimentary formations, which, from oldest to the youngest, are: (a) the Risanguigno Formation (Lower Devonian based on Lazzarotto et al., 2003 and references therein; Middle Mississippian according to Capezzuoli et al., 2021), consisting of marine deposits comprising black phyllite intercalated with dolostone beds, radiolarian chert, green metasandstone, and quartzitic phyllite; (b) the Arenarie di Poggio al Carpino Formation (Late Permian‐Early Triassic), the target of this study, which is composed of quartz‐rich metasandstone, metaconglomerate, and minor metapelite deposited in shallow marine, inner shelf environments and shore deposits; (c) the Civitella Formation comprising green‐gray metasandstone intercalated with lenses of metaconglomerate deposited in a alluvial fan system and belonging to the Verrucano Group (Lower‐Middle Triassic; details and references in Aldinucci et al., 2008; Casini et al., 2008; Conti et al., 1991; Lazzarotto et al., 2003).…”

Alpine subduction zone metamorphism in the Palaeozoic successions of the Monti Romani (Northern Apennines, Italy)

“…However, the recent application of U–Pb detrital zircon geochronology allowed differentiation of Cambrian–Ordovician rocks of Gondwana affinity and late Carboniferous–Permian sequences representing the latest Variscan and earlier post‐Variscan deposits (Musumeci et al, 2011; Paoli et al, 2017). Palinological data from the Palaeozoic successions of southern Tuscany recently provided Carboniferous–Permian ages (Capezzuoli et al, 2021), restricting the occurrence of pre‐Carboniferous sequences to northern Tuscany (Paoli et al, 2017) and the island of Elba (Musumeci et al, 2011) (Figure 1a).…”

“…Other authors correlated these rocks with the Cambrian–Silurian sequences cropping out in Tuscany and Sardinia (Conti et al, 1991; Moretti et al, 1990; Verrucchi et al, 1994). More recently, several Palaeozoic units in the nearby Mid‐Tuscan Ridge (Figure 1a) were attributed to the Permian–Carboniferous post‐Variscan sequences, based on the analysis of fossil palinological assemblages (Capezzuoli et al, 2021) and detrital zircon ages (Paoli et al, 2017). Although no geochronological and palaeontological data exist for the MRC, a similar Carboniferous–Permian age is possible.…”

“…In the Northern Apennines, these parageneses largely occur in the Permian–Triassic metasiliciclastic rocks of the Verrucano Group (Jolivet, Goffé, et al, 1998) that, together with low‐grade Carboniferous–Permian metamorphics, lay tectonically above a Palaeozoic basement that in part registered pre‐Alpine Variscan deformation and metamorphism (Molli et al, 2020; Paoli et al, 2017 and references therein). However, the tectono‐metamorphic evolution of the low‐grade Palaeozoic rocks of the Northern Apennines remains poorly defined, and uncertainty persists regarding their Alpine structural and thermobaric evolution and the role of Variscan inheritance (Capezzuoli et al, 2021; Conti et al, 1991; Franceschelli et al, 1986, 2004; Lo Pò & Braga, 2014; Molli et al, 2020; Paoli et al, 2017). Furthermore, the lack of modern quantitative structural, petrological, and geochronological constraints still hampers a full understanding of the Alpine subduction/exhumation history of the region.…”