Ofioliti, 2004, 29 (2), 285 285 THE EVOLUTION OF THE LIGURIAN UNITS IN SOUTHERN TUSCANY: A NEW PROPOSAL Giuseppe Nirta, Enrico Pandeli and Gianfranco Principi Dipartimento di Scienze della Terra, Università di Firenze, Italy. ABSTRACT To attempt the reconstruction of the stratigraphic – tec- bottom to top of the pile): Castelnuovo Val di Cecina Unit tonic framework of the oceanic units pertaining to the Lig- (CVU), Vara Unit (VU) and Lanciaia Fm., Castelluccio ure – Piemontese Ocean (Ligurian Units Auctt.: Unit (CU), Montignoso Unit (MU). The Lupicaia Creek Helminthoid Flysch Units, Vara Units, Lanciaia Fm.) crop- Unit (LU) is also present locally at the top of the pile (Mon- ping out in Southern Tuscany, sedimentological, biostrati- tecatini Val di Cecina and south of the Montaione-Gam- graphical, petrographical and structural studies were per- bassi). In the Early and to Late Paleocene, the thrust of the formed. These analyses were carried out in some key out- VU onto the Monteverdi M.mo Fm. (CVU) happened and crops (Gambassi-Montaione; S. Donato in Poggio; Northern was sealed by the sedimentation of the Lanciaia Fm. which Monti della Gherardesca; Micciano-Libbiano and Serraz- can be considered an earlier Epiligurian unit. Since Late zano; Lanciaia-Montecastelli Pisano,). Moreover, field data Eocene also the Lanciaia Fm. was deformed by the out-of- were integrated with those coming from the geothermal sequence thrust of the CU and, finally, of the MU. wells. Preliminary structural data point out to two main tectonic The biostratigraphic and petrographic analyses and the events, at least perpendicular, that affect the Ligurian Units. sedimentological observation permit to group the Creta- The first one (axes direction approximately E-W) strongly ceous helminthoid flysches of these areas in a single forma- affects all Ligurian Units while is absent in the Tuscan Units tion, the Monteverdi Marittimo Fm., subdivided in three and in the neogenic sediments. On the other hand the second members: i) Larderello Member deposited in the middle part tectonic event (axes direction NW-SE) affect only the Tus- of the basin,, ii) Montaione Member, deposited in the inner can Unit and the neogenic sediments and refold the first (western?) part and close to an ophiolitic ridge because of event. the occurrence of ophiolitic olistoliths and breccias iii) San In particular, the structural data point out, at least for the Donato Member, that represents the distal eastern part of the Ligurian Units of this sector of the Northern Apennines, to a turbiditic system. structural evolution likely linked, until the Middle – Upper Despite a complex polyphasic evolution, the piling up of Eocene, to a transpressive tectonic context probably focused four main Ligurian tectonic units can be recognized (from along the transform fault of the Ligure-Piemontese Ocean. Presented at the mid-term COFIN-PRIN Project Workshop “Tectonic processes in fossil accretionary prism and comparison with modern analogues: evidences from oceanic units of Northern Apennine, Ligurian Alps and Alpine Corsica”. Firenze, November 2004. .