Geofluid evidence from analysis of deep crustal seismic data (Southern Tuscany, Italy)

Southern Tuscany is characterized by an extensional tectonic regime, crustal thinning, magmatic intrusions, testifying the presence of fluids at elevated temperature and important geothermal fields. The area was investigated by applying integrated methodologies to deep crustal reflection seismic data: tomographic velocity analysis, amplitude versus offset inversion and theoretical approaches to model overpressure conditions. The seismic transect CROP 18, split in two lines (18-A and 18-B) with a total length of 123 km, has imaged the exceptionally high reflectivity of some markers of the metamorphic-crystalline basement units and of the medium-lower crust. The geometry of the main geological structures was reconstructed performing a pre-stack depth migration. The joint analysis of the available data enabled us to distinguish between the lithologic reflectors and the reflections caused by a predominant fluid effect, such as overpressure conditions. We detected the presence of vertical conduits, responsible for magmatic intrusions rising from the base of the crust, as well as of overpressured reservoirs. Theoretical models are used to quantify the effect of overpressure in terms of porosity increase, and the consequent density decrease in two areas where the main marker characterizing the geothermal fields, called the K horizon, is relatively shallow.