Tectonic history of the Monti dell'Uccellina range, Southern Tuscany, Italy

Chara Montomoli(*), Rodolfo Carosi(*) & Piero Carlo Pertusati(*)
(*) Dipartimento di Scienze della Terra, via S. Maria, 53 - 56126 Pisa, Italy. E.mail: montomoli@dst.unipi.it, carosi@dst.unipi.it, pertusati@dst.unipi.it.

Geological mapping, combined with macroscopic and microscopic structural analyses have been used to unravel the geometry and tectonic evolution of the Monti dell'Uccellina group (Southern Tuscany). A polyphase tectonic history has been recognized, characterized by three main tectonic events associated with the development of shear zones, folds and foliations. During the first tectonic phase, East verging F1 folds with axial plane foliation developed under very low-grade metamorphic conditions, as highlighted by illite crystallinity data and by the calcite-dolomite geothermometer. Several piled tectonic units, belonging to the Tuscan and Subligurian domains, constitute the backbone of the Monti dell'Uccellina range. These are, from bottom to top: 1) the Torre Cannelle Unit, made up by the Verrucano Group rocks; 2) the Talamone Unit, made up by the Calcare Cavernoso Formation; 3) the Monti dell'Uccellina Unit, represented by a passive margin sequence spanning from the Triassic Verrucano group at the base to the Tertiary Scaglia Fm. at the top; 4) the Vacchereccia Unit, represented only by Triassic Verrucano Group; 5) the Collelungo Unit, made up by the Calcare Nummulitico Fm. followed by the Macigno Fm; 6) the Canetolo Unit (Sub-Ligurian Unit Auctt.) represented by the «Argille & Calcari» Fm. Stacking of the nappe pile occurred during the first tectonic phase (D1) in two stages: during the first stage the main tectonic units were emplaced. The second tectonic stage is characterized by later thrusts that cut through the older ones, all of them with a top-to-the- East sense of movement, that led to the emplacement of three tectonic complexes. Each complex comprises three superposed tectonic units that are from top to bottom the Collelungo Unit, the Vacchereccia Unit and the Monti dell'Uccellina Unit. During the second tectonic phase (D2), the nappe pile was deformed by a kilometre-scale N-S trending upright antiform gently plunging towards the North. The last tectonic phase is characterized by the development of sub-horizontal folds and of low-angle detachment faults that were produced during the post-collisional extensional events. A detailed survey, that was achieved through compilation of a 1:10.000 scale map, has clarified the structural and stratigraphic position of the «Pseudoverrucano» Auctt. Formation, whose attribution has been debated for a long time. According to our data it can be referred to two different tectonic units: the Vacchereccia Unit, represented only by this formation, and the Monti dell'Uccellina Unit where it constitutes the bottom of a post-Triassic continental margin sequence.

Northern Apennines, Southern Tuscany, thrust tectonics, structural analysis, geological mapping.