UPPER CRUSTAL STRUCTURE OF THE LARDERELLO GEOTHERMAL-FIELD AS A FEATURE OF POSTCOLLISIONAL EXTENSIONAL TECTONICS (SOUTHERN TUSCANY, ITALY)

Reflection seismic profiles crossing the Larderello geothermal field permit the distinction of three different seismic facies units in the upper continental crust. The upper 3-4 km of the crust is characterized by low reflectivity, its base corresponding to a reflector referred to as the K-horizon and recognized in many other areas of Southern Tuscany. Beneath this reflector, a seismic facies of 0.8-1.0 TWT thickness is characterized by strong convergent and divergent reflections forming a lozange-shaped geometry. The lowermost seismic facies is characterized by almost horizontal reflections. Based on the distribution of the hypocentral microearthquakes, temperature data recorded in deep boreholes and the geometry of normal faults, the K-horizon is explained here as a kinematically active rheological boundary which separates a brittle upper part from a ductile lower part. According to this hypothesis, the K-horizon and the lozange reflections could originate from a mylonite band located at the contact between two geological bodies characterized by different rheological behavior. It is inferred that this horizon acts as a decoupling plane in the upper continental crust; it developed as a consequence of the extensional tectonics and the high heat flow which characterize Southern Tuscany.