Age and temperature constraints on metamorphism and exhumation of the syn-orogenic metamorphic complexes of Northern Apennines, Italy.

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. Synorogenic metamorphic rocks are exposed in the mid-Tuscan ridge, Tuscan coast and Tuscan Archipelago and provide information about processes of continental subduction and exhumation associated with accretion and retreat. Apatite and zircon fission-track analyses were applied to some of these syn-orogenic metamorphic rocks. The new low-temperature thermochronological data helped to constrain the timing of the metamorphic peak and the age and modality of exhumation of these rocks that were structurally buried at 20 to 40 km depth during the main tectonogenic phase of the Northern Appenine belt. In the Giglio and Elba islands and in the Monti dell'Uccellina, along the coast, minimum ages of partially reset zircon samples yielded a first time constraint on the Late-Oligocene age of the metamorphic peak. The ca. 27 Ma obtained at the Monti dell'Uccellina is coeval to that found in the Alpi Apuane (located at the northern termination of the mid-Tuscan ridge). The data from the Tuscan Archipelago point to a slightly older age. In the Monti Pisani, south of the Alpi Apuane, and in the Argentario Promontary along the coast, south of the Monti dell'Uccellina, completely reset zircon samples, but composed by different populations, were behaving as multiple thermochronometers and confirm the already proposed isothermal nature of the exhumation occurred after the metamorphic peak. Zircon P1 peak indicated that at 13–10 Ma rocks were already at shallow crustal levels (b9 km). Zircon minimum ages and apatite ages point to a sharp increase in the exhumation rate at ~6–4 Ma. Data indicate a common thermal history for the whole mid-Tuscan ridge and coastal outcrops. The 6–4 Ma increase in exhumation rate can be attributed either to unroofing due to the extensional tectonics or to a contractional “out-of-sequence” or “re-activation” event occurring in the internal sector of Tuscany already affected by a regional extensional regime.