Crustal structures in the geothermal areas of southern Tuscany (Italy): Insights from the CROP 18 deep seismic reflection lines

Two deep seismic reflection lines, named CROP 18A and 18B and roughly NNW–SSE-oriented, were acquired to investigate the crust between the Larderello and the Mt. Amiata geothermal areas, both located in southern Tuscany. Since the Early–Middle Miocene, southern Tuscany has been affected by extensional tectonics, and since the Pliocene, by widespread magmatism. Presently, extension is demonstrated by NE-dipping normal faults and brittle shear zones which affect the crust down to the brittle/ductile transition. In the Larderello area, these shear zones are seismically evidenced by NE-dipping weak reflections and by the loss of the K-horizon reflectivity, a regional mid-crustal high amplitude reflector, located at the top of the brittle/ductile transition. The CROP 18A and 18B lines were reprocessed down to 10 s TWT, down to the crust/mantle boundary. Their reflectivity results are closely related to the direction of the survey: a clear decrease in reflectivity occurs when the survey plane intersects the crustal shear zones along strike. Nevertheless, both lines display segments with low reflectivity and homogeneous acoustic impedance which indicate intrusive magmatic bodies at different crustal levels. A wide, partially crystallised granitoid may be sited at about 7–8 km depth underneath the Larderello geothermal field. In the lower part of the crust, the CROP 18A and 18B transects show group of bright reflections, mainly related to the occurrence of pressurised fluids and/or lithological differences. The base of the crust is located at about 20 km depth.