Alessandra Esposito

Large volume phreatomagmatic ignimbrites from the Colli Albani volcano (Middle Pleistocene, Italy)

Abstract In this paper we describe four large volume, ash- and accretionary lapilli-rich, phreatomagmatic compound ignimbrite units, mafic in composition, from Colli Albani volcano, south of Rome. The four units, that form the ‘Pisolitic Tuffs’ succession, are separated by paleosols and represent the earliest explosive large volume eruptive episodes from the Quaternary Alban Hills volcano. The occurrence of large volume phreatomagmatic–phreatoplinian eruptions implies the availability of large quantities of water interacting with the rising magma. The paleogeography of the area below the volcano has been reconstructed by the analysis of stratigraphic data from more than a thousand bore-holes distributed around the volcano that allowed to identify NW-trending and NE-trending paleotopographic lows that underlie the central area of the volcano and interpreted as extensional tectonic basins. These lows are filled with Lower to Middle Pleistocene, pre-volcanic lacustrine and fluvial deposits and suggest that at least the central part of the Colli Albani volcanic area hosted a large lake or lagoon. The absence of sedimentary xenoliths in the Pisolitic Tuffs and the low vesicularity of scoria and shards suggest that water interacted with a poorly fragmented magma at very shallow level, triggering the large explosivity of the eruptions. We suggest that water interacting with magma was mostly surficial water related to the presence of the lagoon or lake. In this environment, it is likely that vents were subaqueous allowing a continuous access of water to the conduit. Considering that the minimum calculated volume of products for the Pisolitic Tuffs succession is >37 km3 and each eruption unit averages approximately around minimum volume of 10 km3, we suggest that each eruption was related to a caldera collapse, which would have allowed the persistence of a Taupo-like lake in the central area and of phreatoplinian activity. After the last phreatomagmatic eruption, however, the fragmentation style of large volume ignimbrites from Colli Albani became magmatic, suggesting the extinction of the lake after that date. Each of the four units show a basal phreatoplinian fallout level, overlain by a complex association of low aspect ratio surge deposits and ignimbrites. Phreatomagmatic pyroclastic flow deposits are found at distances of more than 40 km from the central area of the volcano, and show important facies variation according to the paleotopography. To the west of the volcano, pyroclastic flows reached the Tyrrhenian coast and emplaced mostly stratified facies on a flat topography, interpreted to reflect both the spreading of pyroclastic flows on an unconfined topography and their interaction with lacustrine and lagoon areas. By contrast, to the east of the volcano, where pyroclastic flows were confined within paleovalleys, the main facies is thick and massive.

The coseismic and postseismic deformation of the L’Aquila, 2009 earthquake from repeated GPS measurements

We analyze more than 100 GPS time series of continuous and discontinuous GPS stations located in the Abruzzi region (Italy) surrounding the epicentres of the L’Aquila 2009 seismic sequence. The purpose of this work is to reconstruct the coseismic displacement field caused by the 6 th April (Mw 6.3) main shock from a dense network of survey-mode stations surrounding the epicentral area and to characterize the early postseismic deformation field. In the months following the main shock, an extensive GPS survey was carried out on the existing Central Apennines Geodetic Network (CAGeoNet), with the intention of collecting a robust data set and to study the co- and postseismic deformation field of this Apenninic normal faulting earthquake. The analysis is carried out with two independent procedures and software (Bernese and Gamit) in order to provide reliable and validated geodetic solutions. The analysis of the postseismic transients and the knowledge of long-term inter-seismic velocities at all GPS stations, issued from permanent and CAGeoNet sites, allow us to derive a dense co- and postseismic displacement field for the L’Aquila Mw 6.3 main shock in a wide area around the epicentre. The highest deformation rate occurs during the first 4–5 months after the main shock and persists in the following at slightly slower rate throughout the whole monitoring period. Fast deformation rates imply that most of the observed deformation is due to a process different from a pure viscoelastic relaxation of the stress perturbation. Since the observed rates would imply a too low effective viscosity value (below 10 17 Pa s), we rather suggest that most of the observed deformation in the first months after the earthquake is due to different processes, most likely frictional afterslip possibly modulated by the presence of fluids. The new coseismic displacement field is used to invert for the main shock fault geometry, analysing the consistency among the different geodetic solutions and the combined one, with the goal of validating the two data sets.

Sannio-Matese Mounts (Southern Italy) deformation field from GPS Data (2002-2014)

Lucchi, Renata G. ... et. al.-- 87° Congresso della Societa Geologica Italiana e 90° Congresso della Societa Italiana di Mineralogia e Petrologia, The Future of the Italian Geosciences - The Italian Geosciences of the Future, 10-12 September 2014, Milan, Italy.-- 1 pageThe Montellina Spring (370 m a.s.l.) represents an example of groundwater resource in mountain region. It is a significant source of drinking water located in the right side of the Dora Baltea Valley (Northwestern Italy), SW of Quincinetto town. This spring shows a morphological location along a ridge, 400 m from the Renanchio Torrent in the lower sector of the slope. The spring was investigated using various methodologies as geological survey, supported by photo interpretation, structural reconstruction, NaCl and fluorescent tracer tests, discharge measurements. This multidisciplinary approach, necessary due to the complex geological setting, is required for the importance of the Montellina Spring. It is interesting in the hydrogeological context of Western Alps for its high discharge, relatively constant over time (average 150 l/s), and for its location outside a fluvial incision and suspended about 40 m above the Dora Baltea valley floor (Lasagna et al. 2013). According to the geological setting, the hydrogeological reconstruction of the area suggests that the large amount of groundwater in the basin is essentially favoured by a highly fractured bedrock, covered by wide and thick bodies of glacial and gravitational sediments. The emergence of the water along the slope, in the Montellina Spring, is essentially due to a change of permeability between the deep bedrock and the shallow bedrock and/or surficial sediments. The deep bedrock, showing closed fractures and/or fractures filled by glacial deposits, is slightly permeable. The shallow bedrock, strongly loosened as result of gravitational phenomena, and the local gravitational sediments are, on the contrary, highly permeable. The concentration of water at the spring is due to several reasons. a) The spring is immediately downward a detachment niche, dipping towards the spring, that essentially drains the water connected to the change of permeability in the bedrock. b) It is along an important fracture, that carries a part of the losses of the Renanchio Torrent. c) Finally, it is favored by the visible and buried morphology. Although it is located along a ridge, the spring occurs in a small depression between a moraine and a landslide body. It also can be favored by the likely concave trend of buried base of the landslide. At last, tracer tests of the Renanchio Torrent water with fluorescent tracer are performed, with a continuous monitoring in the Montellina Spring. The surveys permit to verify and quantify the spring and torrent hydrogeological relationship, suggesting that only a small fraction of stream losses feeds the spring.