THE MECHANICS OF THE CAMPI-FLEGREI RESURGENT CALDERA - A MODEL

A model of caldera resurgence has been applied to the Campi Flegrei (southern Italy) to interpret: (a) the processes observed during the recent bradyseismic crises of 1970-1972 and 1982-1984; (b) ground movements recorded for the past 2000 years; and (c) the volcanic history of the area. The two-dimensional mechanical model involves an area larger than the caldera itself. The crust is considered as a viscous plate overlying a viscous half-space. Geometric dimensions and mechanical parameters are the assumed variables and have been assigned a range of values of 50-100 km for length, 5-15 km for thickness, and 10(21) to 10(22) Pa s for viscosity. The process controlling volcanic activity in the Neapolitan area is attributed to a tensile stress field causing the opening of the Tyrrhenian Basin and the rising of the mantle to depths of about 15-25 km beneath Campi Flegrei. While regional tumescence can be related to magmatic upwelling, detumescence can be linked to the major eruptions in the area (the Green Tuff of Epomeo, Ischia, 55,000 yr B.P.; the Campanian Ignimbrite, 30-35,000 yr B.P.; and the Neapolitan Yellow Tuff, 12,000 yr B.P.), together involving more than 100 km3 (DRE). We propose that regional detumescence is likely to have triggered caldera resurgence. Indeed, the model suggests that detumescence may have generated an axial tectonic stress of about 1000 bar (100 MPa), a value which is of the correct order of magnitude for initiating caldera resurgence in the Campi Flegrei. Furthermore, we show that such a stress decreases the vertical pressure needed for uplift to occur.