Vein development and fluid inclusion data: insight on the evolution of the Tuscan Nappe in the Northern Apennines

Meso to microstructural analyses performed on the Tuscan Nappe outcropping in the La Spezia area (Northern Apennines), pointed out the presence of a polyphase tectonic history characterized by three tectonic phases all of them associated to the development of folds. A detailed deformation study on syn-tectonic vein arrays was carried out in order to better constrain the tectonic evolution of the area. To characterize the thermobaric evolution of the Tuscan Nappe during the late stages of its tectonic evolution, a fluid inclusion study was performed on syn-D2 quartz-bearing fibrous veins. The second tectonic phase, recognized for the first time during this work, is associated to the development of upright, centimetric to kilometric in size, F2 folds. Fluid inclusion data highlight the presence of a retrograde metamorphic pattern, with respect to the D1 syn-metamorphic tectonic phase, suggesting P-T conditions ranging from 250 MPa and 280°C to 110 MPa and 160°C. F2 fold geometries suggest that they developed in a compressive tectonic regime characterized by a retrograde metamorphic pattern, accompanied by isothermal pressure drops. These data point out that the recognized compression-related D2 tectonic phase, was related with the beginning of the exhumation of the Tuscan Nappe, whereas extensional tectonics postdates D2 deformation phase and played a major role later in its tectonic history