| Abstract |
The Monte Amiata volcano consists of acidic and intermediate products such as dacitic, rhyodacitic and olivine-latitic rocks containing mafic enclaves. This volcanic complex derived from the emplacement at a depth of about 6-7km of a broad anatectic magmatic intrusion, considered at the origin of the current geothermal anomaly of the two fields under the exploitation of Bagnore and Piancastagnaio. These volcanic rocks host an important phreatic aquifer in southern Tuscany. At its base this aquifer is confined by very low permeable rocks consisting of shales, sandstones and Ophiolite relics (Ligurian Units) which have recorded the whole tectonic history of the Northern Apennines orogenesis. Such rocks separate this aquifer from the geothermal reservoir hosted within permeable horizons at deeper levels. One of the most important items presently under discussion is the shape and origin of the base of the volcanic complex, i.e. the bottom of the volcanic aquifer. Borehole data, drilled for geothermal, water and mining purposes, have been integrated with geophysical interpretation of VES (Vertical Electrical Sounding) and EM soundings (Electro-magnetic) in order to reconstruct the geometry of the geological surface separating volcanites from underlying pre-volcanic rocks. The obtained surface has been compared with the DEM (Digital Elevation Model) of the surrounding area, by means of GIS (Geographic Information System) techniques, and with the geological setting of the volcanic substratum, revealing an abundance of information on the likely landform characterising the Monte Amiata geothermal region prior to the volcanic eruption. It has been highlighted that the morphology of the pre-volcanic substratum, hidden below the volcanic complex, fits well with the present landform in the surroundings of the volcano and the results are strongly influenced by the structural heritage deriving from the Neogene-Quaternary tectonic evolution of the Northern Apennines. As a whole, a good correlation has been observed between the morphological high locations and the laterally segmented geological bodies located at depth within the substratum as they have been hypothesized in literature. These geological bodies are related to the activity of Middle-Late Miocene low-angle normal faults which affected the Mt. Amiata region, as well as the whole inner Northern Apennines, after the building of the orogenic edifice. In conclusion, the geometry of the base of the Mt. Amiata volcanic complex seems to preserve a paleomorphology derived from the erosion of the prevolcanic uplifted substratum, thinned by extensional structures since Middle-Late Miocene. |