Intrusion and deformation at Campi Flegrei, southern Italy: Sills, dikes, and regional extension

[1] The Campi Flegrei volcanic district, in southern Italy, has been uplifted since 1968 by a net maximum of 3 m during the intervals 1968–1972 and 1982–1984. The uplift represents a permanent deformation against a background rate of subsidence of about 17 mm a⁻¹. Previous models have reproduced the observed vertical deformation but not the full pattern of horizontal movements. The 1982–1983 deformation is here reanalyzed in terms of a penny-shaped sill on its own, with a tabular surface protrusion, or in an extensional stress field. It can be explained best by the intrusion of a sill (of 0.03–0.04 km³ at a depth of 2.75 km) in a crust that is being stretched ESE-WNW at a strain rate of about 5.6 × 10⁻⁵ a⁻¹. The sill's volume is similar to the common volumes of Campi Flegrei's eruptions since the Neapolitan Yellow Tuff (NYT) caldera was formed 15.6 ka ago. This similarity and the permanent nature of the uplift favor magmatic intrusion as the primary source of unrest. Sill formation may thus reflect the spreading of magma at a level of neutral buoyancy or along lateral discontinuities in the crust. The southern part of the caldera has been shielded from post-NYT eruptions, despite some 33 m of permanent uplift since Roman times. Precursors to eruptions may thus be related not to caldera-wide uplift but to the preceding conditions that determine whether magma ascends beneath the southern part of the caldera (favoring sill intrusion) or elsewhere (favoring an eruption).