The volcanic and geothermally active Campi Flegrei caldera: an integrated multidisciplinary image of its buried structure

The Campi Flegrei caldera in southern Italy is one of the greatest geohazard areas on Earth. Evidence of an active magmatic and geothermal system is provided by ongoing ground uplift, with volcano-tectonic and long-period (LP) seismicity, the persistent degassing of similar to 1500 tonnes of CO2 per day, the presence of hot fumaroles at temperatures of 90-150 A degrees C, brine-rich aquifers (with total dissolved solids up to 33 g l(-1)) and high thermal gradients in the crust (with temperatures reaching 420 A degrees C at 3,050 m b.s.l.). Since the 1940s, more than 100 exploratory boreholes have been drilled in the area to depths of 80-3,100 m by the Azienda Geologica Italiana Petroli (AGIP) and the SocietA Anonima Forze Endogene Napoletane (SAFEN). To date, however, no systematic reanalysis of the drilling data has been carried out, and the buried volcanic structure has not been updated using the most recent scientific results and previous findings. By integrating unpublished data from the AGIP and SAFEN reports with published information from geological, volcanological, petrological, petrophysical and geophysical studies, this paper presents an improved picture of the Campi Flegrei caldera that will be useful for volcanic hazard assessment and mitigation in the Naples area and for future research planning. The results suggest that intra-caldera activity has been influenced by how the magmatic system at depths greater than about 4 km has determined the transfer of magma, volatiles, and heat to the overlying geothermal system and, ultimately, to the surface. In particular, intriguing is that the most volcanically active central-eastern sector of the caldera, which is subject to intense bradyseismic ground movement and gas emission, coincides with a structurally delimited subsurface rock volume characterized by an uprising of the 100 A degrees C isotherm, a deep water supply to the shallower aquifer, the early disappearance of secondary calcite, LP seismicity and high seismic S-wave attenuation. In this area, we also document evidence of repeated injection at depths of c. 1.5-3.0 km of isolated and small-volume batches of magma, where occurred their crystallization and degassing. Shallow intrusions and degassing of magma are thus identified as two of the key processes that drive unrest in Campi Flegrei.