STRUCTURAL EVOLUTION OF CAMPI FLEGREI CALDERA IN LIGHT OF VOLCANOLOGICAL AND GEOPHYSICAL-DATA

The structural evolution of Campi Flegrei (CF) has been investigated by combining on land and offshore gravimetric and aeromagnetic data, with surface geological and volcanological evidences and subsurface drilling information. A relatively small (4-5 km across) collapsed circular structure, produced by eruption along a ring fracture, has been identified for the first time in the Agnano-Astroni-Solfatara (AAS) sector. Data permitted the definition of the geometry of large CF caldera, which is 13 km across and includes an inner more collapsed zone, 11-12 km across with a total sinking of about 1,600 m. Its volume (80 km3) is consistent with that estimated for the Campanian Ignimbrite erupted 35,000 a B.P.-A further collapse occurred after the eruption of the Neapolitan Yellow Tuffs (10 km3; about 12,000 a B.P.) and finally the AAS structure was formed in the last phase of activity, about 4,000 a B.P.-The CF volcanic and structural evolution is therefore characterized by the repetition of a similar eruption mechanism, which includes the formation of a ring fracture, magma uprise and eruption from several points along the fracture and collapse of the central sector into the emptied magma chamber. The decrease with time of the dimensions of the resulting structure (diameter of the collapsed area; total sinking) reflects the progressively decreasing volume of the magma available in the chamber.