New data on the paleogeography of Southern Tuscany (Italy) since Late Miocene time
In order to add new data to the knowledge of the paleogeography of Southern Tuscany and the Tuscan shelf, a multidisciplinary study on the petrology and distribution of magmatic clasts of the widely exposed Messinian, Pliocene, and Quaternary sediments in Southern Tuscany was carried out. The magmatic clasts consist of porphyric aplites and subordinate granite porphyries, which derive from eroded subvolcanic acidic bodies. The porphyric aplite clasts were analyzed in detail to define their textural, petrographic, and chemical (major and trace elements, Sr and Nd isotopic composition) features. The porphyric aplite clasts show strong affinities with the 8.4–7.9 Ma old Capo Bianco aplite, whereas the granite porphyry clasts have affinities with the Portoferraio porphyry (ca. 8 Ma) which intruded the Cretaceous and Paleogene Flysch Units and the Ophiolitic Unit in central-western Elba Island (Northern Tyrrhenian Sea). The present outcrops of the Capo Bianco aplite in central Elba Island cannot be considered as the source rock for the Late Messinian gravels, because at that time such Capo Bianco aplite (located at the lower portion of the laccolitic stack) was there buried at several kilometers depth. However, other Capo Bianco-like aplitic bodies outcropped during the latest Miocene in the host rocks above and also around the 6.8 Ma M. Capanne Pluton in the western Elba areas. The exposure of such bodies was made possible by the activation of the central Elba detachment fault (=CEF), due to the uplift of the M. Capanne pluton. This uplift displaced a significant portion of the cover (including the upper portion of the laccolithic stack) of the granitoid body at about 6, 7 Ma, thus allowing the erosion of the lower part of the laccolith complex made up of the Capo Bianco aplite and of the Portoferraio porphyry and leaving still buried the M. Capanne pluton. The paleogeographic picture of Southern Tuscany arising from the collected evidences is the following: during Late Messinian, the clasts were dispersed by a SSW/NNE-trending complex fluvial system in the Colline Metallifere area. The Early Pliocene extensional tectonics cut off the detrital inputs from the Messinian source areas, because of the onset of the Piombino Channel and of the Campiglia–Gavorrano Ridge. During the Pliocene, the clasts were cannibalized from the previous sedimentary units and reached the easternmost areas due the eastward progressive uplift of the Colline Metallifere, likely connected with the coeval magmatic intrusions. Finally, the Quaternary regional uplift allowed a drainage reversal and a backward displacement of the aplitic clasts toward the Tyrrhenian coast. These data point to a rapidly evolving drainage pattern in Southern Tuscany during the considered time interval, which was mostly driven by the intrusion and uplift of the Messinian to Quaternary plutons. The morpho-tectonic evolution is well framed also within the models since long accepted for the Northern Apennine geodynamics, characterized by an overall eastward shift of the orogenic front.