2-D Thermal Modelling Across the Geothermal Fields of Tuscany, Italy.

The lithospheric extension affecting the Tuscan-Tyrrhenian domain represents one of the most relevant and recent tectonic processes within the entire Alpine-Mediterranean deformation area. The heat input from the mantle is responsible for the presence of large geothermal resources at accessible depths in the crust, as testified by temperature and heat flow anomalies, locally extremely high. Despite intensive exploration and exploitation drilling programs carried out in Tuscany, mainly since the 70s, the nature, physical properties and structure of the intermediate and lower crust and of the upper mantle are still debated. The available dataset for the Tuscan area was significantly improved by the acquisition of the deep crustal seismic reflection profiles (CROP Profiles), in the mid 90s. The profiles CROP 18A and 18B, crossing NW-SE wards the Larderello and Monte Amiata geothermal fields, and the CROP 03 profile, intersecting with W-E direction the CROP 18B, were recently reprocessed to better characterize the crustal and upper mantle structures of the entire Tuscan geothermal area. The results show new remarkable and interesting features, i.e.: the presence of extensional structures below the "K Horizon" regional high-amplitude discontinuous reflector; a second deeper and more continuous similar horizon; mantle intrusions; strong reflectors in the lower crust and a discontinuous crust/mantle transition with possible underplating. Accurate analyses of the seismic attributes suggest the presence of fluids/melts from the "K Horizon" down to about 10 km depth. These new data were put into a simple and conceptual 2-D model, aimed to provide a set of preliminary thermal models, to be compared with the experimental borehole temperature and heat flow data. The 2-D numerical modelling followed a two-steps process: first, modelling the regional conductive heat transfer in the upper 10-12 km of the crust and, secondly, superimposing local advection, in correspondence of the geothermal fields. The 2-D regional conductive model was realized by means of a steady-state forward simulation, under the assumption of a purely conductive heat transfer. The unknowns are the basal heat flow and the thermal properties of the crustal rocks, whereas the results are the temperature distribution with depth and the surface heat flow. To account for the uncertainties in the physical properties of the crustal rocks we produced two sets of models, using the parameters and assumptions which maximise and minimise, respectively, the surface heat flow output to be compared with the experimental data. Local heat transfer by advection was introduced in the upper crustal structures of the geothermal fields, where the CROP seismic profiles were indicating presence of fluids. The temperature, depth and extension of these reservoirs can explain most of the present extremely high surface heat flow anomalies.