A new gravity map in a densly inhabited area (Ciampino and Marino Districts) of Colli Albani Volcanic District characterized by the occurrence of geochemical manifestation

Abstract

The Colli Albani is a quiescent Volcanic District, located almost 20 Km southeast of Rome. It has been active since at least 600 ka and it belongs to the potassic and ultrapotassic Roman Magmatic Province, a northwest-trending chain of volcanoes that developed along the Tyrrhenian Sea Margin of Italy during middle and late Pleistocene time. The volcanic history is dominated, in the first time, by the explosive eruptive activity (Tuscolano-Artemisio Phase), that formed the caldera; after a years of dormancy the activity took place in the centre of the caldera, Monte delle Faete Edifice, was characterized by effusive and strombolian activities. The Albano maar (< 70 ka) represents the most recent activity of the hydromagmatic phase and cannot be considered extinguished yet. Infact, at present, the area is characterised by almost continuous low-level seismic activity (Amato et al., 1994, 1995) and by the presence of an intense deep CO2 degassing process, which causes widespread gas emission (CO2 and H2S), and numerous soda water discharges (Chiodini & Frondini, 2001). A broad area N of lake Albano (Rome, Ciampino-Marino) is the one most strongly characterized by the occurence of geochimical manifestation of a still-active subvolcanic system which are hazardous factors for the resident population. This anomalous degassing area was found to be coincident with positive gravity anomaly (“Ciampino high”, Di Filippo & Toro, 1995), drawing to the surface, deep structures with its geometry. In order to define better the deep geological and structure setting between Ciampino and Marino Districts, gasses are able to reach a suitable migration rate for the formation of soil-gas anomalies throw faults and fractures, and geological distribution in sub-surface (sedimentary substratum, mainly consisting of marine sand and sandy clays of Plio-Pleistocene age, who filled the extensional basis, and Mesozoic-Cenozoic carbonaceous-marly-siliceous succession from Umbro-Sabina Series), a new gravity map was costructed using data from the 1:100,000 Gravimetric Map of the Italian Geological Service, gravity surveys performed by Di Filippo & Toro in 1995 and from recent gravity surveys performed by the authors that included 300 new survey stations. On the basis of this information a new residual anomaly map has been prepared by substracting the first order Regional Field from Bouguer anomalies. The residual anomaly map is very detailed and represents the most suitable picture for understanding the structure of the prevolcanic substratum. The superficial geological data, the drill log data, and metamorphic xenoliths from the eruption of Colli Albani (Funiciello & Parotto, 1978) and gravity data have been analyzed all together in order to know sub-surface structure of this area and make a three-dimensional gravity model. In order to define the heterogeneity of the lithotypes some sections have been prepared by gravity model.