Abstract A 120 Km-long deep seismic reflection survey, named CROP 18, was acquired during 1995-1998 to investigate the crust structures of southern Tuscany (inner northern Apennines) geothermal areas. The survey is separated into two transects, passing over the Larderello and Monte Amiata geothermal fields, respectively. Extensional structures characterise the geological setting of southern Tuscany continental crust. Extension has been affecting inner northern Apennines since Early-Middle Miocene times. During the Miocene, a widespread boudinage of the previously stacked tectonic units occurred and determined tectonic depressions where continental and marine sediments deposited. The boudinage was followed by Pliocene-Quaternary normal faults that, in the Larderello area, coalesce in brittle shear zones which sole out into the present brittle-ductile transition. The intersection between the shear zones and the rheological boundary is marked by the loss of the K-horizon reflectivity, a high amplitude regional seismic reflector. The CROP 18 plane, alongstrike oriented, intersects the Pliocene-Quaternary shear zones. This implies that the CROP 18 reflectivity is generally weak down to the K-horizon depth. In contrast, the reflectivity below the K-horizon is better imaged. The CROP 18 survey reveals a continental crust affected by magmatic intrusions which have been developing since Pliocene, at least. The high heat flux, typifing the geothermal areas could be related to partially crystallised magmatic bodies, sited at mid-crustal levels.
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