Crustal models of the geothermal areas of southern Tuscany (Italy).

Geological and geophysical data gathered in the geothermal areas of southern Tuscany, located in the inner part of the northern Apennines, indicate the presence of granite intrusions. The most relevant data are: (1) regional asthenospheric ascent to a depth of 40-50 km; (2) granitic dykes as old as 3.8-2.9 Ma and a widespread contact aureole found in several deep geothermal wells; (3) very high heat flow values, interpreted as the result of convective circulation of hot fluids above shallow magmatic bodies; (4) the presence of low-density bodies inferred from low gravity values, often correlated with positive heat flow anomalies; (5) the occurrence of a highly reflective horizon (named K) in a depth range of 3-12 km in the geothermal areas as shown by seismic reflection data; (6) a 30-40 km wide low-velocity body, below the K horizon, whose existence is supported by local earthquake tomography and teleseismic studies; (7) the presence of a conductive body which correlates well with the anomalous tomographic and teleseismic body, as revealed by magnetotelluric surveys. The goal of the paper is to test different alternative crustal models by using these data. The models are (1) granite intrusion, (2) geopressurized fluids below impervious layers, (3) contact metamorphic aureole, (4) mylonite layers at the upper/lower crust transition, and (5) some combination of the four. The occurrence of shallow intrusive bodies, still partially molten and associated with saline brines is considered the most tenable model.