Chronostratigraphy and paleomagnetism of Oligo-Miocene deposits of Corsica (France) : geodynamic implications for the liguro-provençal basin spreading

In a context of convergence between Africa and Europe, the western Mediterranean domain is characterized by the opening of the Liguro-Provençal ocean in the early Miocene and of the Tyrrhenian Sea since the middle Miocene. These openings are preceded by an Oligocene rifting episod. New biostratigraphic, geochronologic and paleomagnetic data allow to propose an integrated stratigraphy sketch for the Oligo-Miocene period in Corsica. The continental syn-rift deposits, in the region of Ajaccio, have been dated as late Chattian from the presence of a mammal, Pomelomeryx boulangeri. Their paleomagnetic study indicates an anticlockwise rotation of 44 ±4° with regard to the stable Europe. The early Miocene succession was deposited on a contrasted topography inherited from the Oligocene glyptogenesis period. The lowermost Miocene deposits are represented by 4 pyroclastic flow deposits in southern Corsica. New 40Ar-39Ar datings on plagioclase concentrates give ages between 21.3 and 20.6 Ma (late Aquitanian). Paleomagnetic directions indicates that the rotation of Corsica had probably begun at this time. The Miocene marine sediments of the Bonifacio basin are divided in two formations. At the base, the Cala di Labra formation, that comprises 3 reef formations in coastal onlap (R1, R2, R3), is attributable to the Globigerinoides trilobus zone. In the environment of R1, miogypsin population includes mainly Miogypsina intermedia with a few M. globulina and M. cushmani. The absence of the less evolved forms (10 < V < 35) let us suppose that at least the lower Burdigalian is absent at Bonifacio because the region was emerged. R2 reef has a similar population (42 < V < 70), which suggests a rather short interval of time between the two stages of reef construction. On the other hand the R3 population contains only evolved forms (M. mediterranea) suggesting a late Burdigalian age. The end of the sedimentary succession (Bonifacio formation) is essentially represented by a pile of hydraulic dunes. The absence of Orbulina shows that this upper unit is older than late Langhian (biozone N9). Miocene sediments from Saint-Florent are divided into 3 marine formations bracketed by two continental ones. At the base, the continental formation of Fium’Albinu (equivalent to the Cala di Labra formation) is surmounted by the Torra formation attributed to Late Burdigalian based on the presence of G. trilobus and G. bisphericus. The Sant’Angelo formation (equivalent to the Bonifacio formation) developed from latest Burdigalian to late Langhian. The Farinole formation represents early Serravallian (base of the Globoquadrina altispira altispira zone). Despite the large number of studied sites in this study and others, the only reliable Oligo-Miocene paleomagnetic results in Corsica are the following ones : – the Vazzio formation indicates a rotation of 44 ± 4° after the late Oligocene, – the southern volcanic flows show that the rotation was in progress around 20.7 Ma (late Aquitanian), – the Fium’Albinu formation indicates a rotation of 30 ± 12° after early Burdigalian, – site B from Vigliotti and Kent [1990] gives a rotation of about 11 ± 5° after early Langhian. Our paleomagnetic data, together with the existing ones, associated to the biostratigraphic and geochronologic ages show that the opening of the Ligurian basin coincides with an anticlockwise rotation of 45° of Corsica, which begins between 23 and 21 Ma and ends around 15 Ma. This is in agreement with the idea of Sardinia and Corsica rotating as a single almost rigid block during Miocene times.