Alfio Viganò

Geothermal and rheological regime in the Po plain sector of Adria (Northern Italy)

We present a thermo-rheological study of the Adriatic crust in the central Po plain (Northern Italy), which records the convergence of the Alpine and Apennine external fronts. The present thermal regime of the crust is inferred from geological and geophysical data from oil exploration. A set of temperature data from 37 deep boreholes (bottom hole, drill stem, and production test temperatures) is used to estimate undisturbed formation temperatures down to 6-7 km of depth. Best quality data are used to estimate upper crustal geotherms, which are subdivided into four groups taking into considerations the following factors: i) presence/absence of relevant fluid circulation; ii) conductive thermal refraction; iii) recent sedimentation, and iv) other disturbing tectonothermal processes (e.g., recent tectonics). Assuming average corrections for temperature data affected by recent sedimentation effects, geotherms fall within the same range and provide a homogeneous thermal signal. Temperature measurements in the upper ~7 km are used to constrain a 1-D thermal model at the crustal scale. The best fitting geotherm gives a thermal gradient of ~23 ± 5 mK m-1 (over 7 km of depth), a surface heat flow of ~46 ± 9 mW m-2, and an estimated Moho temperature of 620 ± 80 °C.Using the estimated crustal structure and composition and the best-fitting geotherm, a representative strength envelope is obtained.For hydrostatic and supra-hydrostatic pore pressures, the calculated total crustal strength of this sector of the Adria microplate (not including the upper mantle contribution) is in the 1.8-2.5 TN m-1 range. Considering also the lithospheric mantle contribution, the total lithospheric strength is estimated to be between 23 (wet peridotite)and 39 (dry peridotite) TN m-1.

Geomorphology and Age of Large Rock Avalanches in Trentino (Italy): Castelpietra

Within a project aimed at understanding past catastrophic rock slope failure in the Trentino Province of Italy, we studied the Castelpietra landslide. Castelpietra encompasses a main blocky deposit, with an area of 1.2 km2, which is buried on the upper side by more recent rockfall debris. The release area is the Cengio Rosso rock wall, which is comprised of Dolomia Principale and overlying Calcari Grigi Group dolomitized limestones. 36Cl exposure dates from two boulders in the main blocky deposit indicate that the landslide occurred at 1060 ± 270 AD (950 ± 270 yr ago). The close coincidence in time of the Castelpietra event with several events that lie within a maximum distance of 20 km, including Kas at Marroche di Dro, Pra da Lago and Varini (at Lavini di Marco) landslides, strongly suggests a seismic trigger. Based on historical seismicity compilations, we have identified the “Middle Adige Earthquake” at 1046 AD as the most likely candidate. Its epicenter lies right in the middle of the spatial distribution of the discussed landslides.

Pseudotachylytes of the Tonale nappe (Italian Alps): petrogenesis, 40Ar-39Ar geochronology and tectonic implications

Pseudotachylyte veins of the Tonale nappe (NE Italy) occur along faults and shear zones which accommodated the deformation that shortened the Adriatic crust to the north of the Periadriatic Lineament during the late Alpine orogeny. These veins are characterized by thin glass layers showing variable clast content and colours, coupled with chemical variations. They are derived from surrounding gneiss, but mass-balance studies show some quartz missing in the veins (i.e., 1 rock = a vein + b quartz, with b ranging from 0.1 to 0.3). Thermodynamic modelling of the melting process indicates that the energy necessary to melt 1 cm3 of rock is 4-5 kJ. Temperature reached inhomogeneous values between ~1100°C (total melting of micas) and ~1700°C (partial melting of the quartz clasts). The modelling of the cooling process accounts well for the microlite blastesis in the glassy groundmass of the veins, provided that the pressure at which pseudotachylytes formed along faults/shear zones ranges between 4 and 8 kbar. 40Ar-39Ar laser-probe in-situ data show that vein zoning corresponds to a range of apparent ages. Step-heating analyses indicate the presence of components younger and older than the age proposed for the vein, due to alteration and clasts, respectively. Interplay between these two factors affects the in-situ data. The new 40Ar-39Ar ages of the Tonale nappe pseudotachylytes (~50-21 Ma), as compared to previously published data, testify a long-lived deformation lasting from Eocene to Early Miocene. The peak of this deformation is Late Eocene-Early Oligocene in age (~35-30 Ma), coeval with the emplacement of the Northern Adamello plutons and of several apophyses and dykes in the Adriatic crust.

Reconstruction of the 550 x106 m3 Molveno rock avalanche

With an estimated source volume of 550 x106 m3, the Molveno rock avalanche is one of the largest in the Trento Dolomites. Two source areas have been suggested one to the west at Mt. Soran and one to the east at Mt. Gazza. Conversely, some authors suggest the presence of two landslides one from each of the source areas. Rock avalanche debris dammed the gorge between the Molveno Valley to the north and the Nembia Valley to the south forming the Molveno Lake (823 m a.s.l.) (Sauro and Zampieri, 2001; Chinaglia and Fornero, 1995). Here we present results of a study aimed at better constraining the events leading up to the emplacement of this massive deposit on the valley floor. We improve constraints on the extent and volume of the deposit using a combination of GIS, field mapping, and literature review (including results from earlier geophysical investigations). This is then compared to volume calculations from the preferred Mt. Soran source area using a 3D model developed using a combination of Google Earth and standard GIS tools. In order to improve confidence in our assumed source area, we transfer our modelled pre-failure topography to DAN3D, a continuum dynamic model designed to analyze the runout of highly mobile landslides. Although calibrating the parameters for DAN3D requires a back-analysis of the landslide runout, we obtain a reasonably good fit between the modelled and observed deposits using our assumptions of a single event from Mt Soran. Surface exposure dating from a number of boulders located on top of the deposit allow us to better constrain the timing of the event.

Integrated geophysical-geological model for seismic local site response: the Caldes alpine slope case (Southern Alps, NE Italy)

We present the seismic local response analysis of an alpine slope based on an integrated geophysical and geological model. The studied site is located in Caldes, in the lower Val di Sole valley in the Trentino region, Southern Italian Alps. Caldes represents the typical alpine valley formed by glacial and fluvio-glacial processes characterized by complex geomorphological, lithological and tectonic settings. A detailed reconstruction of the valley slope geology (subsoil geometry and stratigraphy) was obtained using electrical tomography resistivity soundings, controlled source multi-channel analysis of surface waves, microtremor single station measurements, borehole investigations, and geotechnical analyses on undisturbed samples. The geological model was shaken with real accelerometric series to obtain 1D (linear equivalent approach) and 2D (finite element approach) local seismic responses. The results, considering the uncertainties of the geophysical measurements, show different seismic responses between 1D and 2D approaches due to relevant geometrical amplifications, with clearer variations in the 2D approach. The comparison of the results with the building code design spectra shows the limits of the simplified approaches, suggesting that detailed characterizations are necessary, especially for such populous alpine environments.