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 p ermit the distinction of three different seismic facies units in the u pper continental crust. The upper 3-4 km of the crust is characterized by low reflectivity, its base corresponding to a reflector referred t o as the K-horizon and recognized in many other areas of Southern Tusc any. Beneath this reflector, a seismic facies of 0.8-1.0 TWT thickness is characterized by strong convergent and divergent reflections formi ng a lozange-shaped geometry. The lowermost seismic facies is characte rized by almost horizontal reflections. Based on the distribution of t he hypocentral microearthquakes, temperature data recorded in deep bor eholes and the geometry of normal faults, the K-horizon is explained h ere as a kinematically active rheological boundary which separates a b rittle upper part from a ductile lower part. According to this hypothe sis, the K-horizon and the lozange reflections could originate from a mylonite band located at the contact between two geological bodies cha racterized 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.