TERTIARY EXTENSIONAL TECTONICS IN TUSCANY (NORTHERN APENNINES, ITALY)

Two extensional events have been recognized in the Tertiary evolution of Tuscany. The first event occurred in the period between the Late Oligocene and Late Tortonian and caused a stretching of 120% or greater; the second event occurred between the Late Tortonian and Quaternary Periods and caused a stretching not greater than 10%. The first event is characterized by the exhumation of middle crust rocks and therefore by the development of core complexes. The most significant structures of the first event are: low-angle normal faults, which characterize the upper plate of the core complexes, and ductile shear zones, which characterize the lower plate. The low-angle normal faults tend to die out in the Late Triassic evaporite level, which constitutes the main detachment of the core complexes. The main structures of the second extensional event are high-angle normal faults which dissect all the previous tectonic features. These faults border tectonic depressions which were the site of elastic sedimentation from Late Tortonian to Quaternary time. These normal faults may tend to flatten in a seismic lozenge-shaped band explained as an active extensional shear zone, which is located at the brittle/ductile boundary. A similar explanation is given for the detachment located in the Late Triassic evaporite level during the first extensional event. While the first extensional event is in the framework of the Northern Apennine post-collisional evolution, the second extensional event, because it affects an already stretched crust, could be attributed to the southern Tyrrhenian opening, which began during Late Tortonian.