3D reconstruction of a polyphase thrust stack in the sedimentary cover of the Southern Alps (Italy)

The realization of a 3D geological model needs detailed geological maps that must be integrated with field observation and measures on the main surfaces that are reconstructed in the model. The 3D geological model of a thrust stack in the central part of the Orobic Alps has been reconstructed integrating geological data from the 1:10,000 geological GIS data base realized by Regione Lombardia in the frame of the new Geological Map of Italy 1:50,000 (CARG project) with new data. The study area, about 100 Km2 wide, is characterized by sedimentary rocks ranging in age from Late Permian (Verrucano Lombardo) to Norian (Dolomia Principale). The structural setting of the area is complex: northward, an important fault (Valtorta-Valcanale Fault) divides the Triassic from the Permian succession, whereas southward another fault (Clusone Fault) separates Carnian from Norian units. Between these faults, the succession is tectonically repeated building up an antiformal stack. The lithostratigraphic units (total thickness about 1500 m) are represented by subtidal and peritidal carbonates (Angolo Limestone, 250-300 m thick), basinal marly limestones (Prezzo Lmst., 10-15 m), carbonate platform facies (Esino Lmst., 700-800 m) characterized at the top by an erosional surface covered by peritidal (Breno Fm.) and later subtidal (Calcare Metallifero Bergamasco) limestones (80-100 m), capped by 150 to 300 m of lagoonal marls and limestones (Gorno Fm.) and deltaic sandstones (Val Sabbia Sandstone). The succession ends with terrigenous-carbonate sabkha facies of the S. Giovanni Bianco Fm. (about 100 m preserved). The 3D reconstruction of the main thrust surfaces and of the lithologic boundaries has been achieved through the elaboration of CARG data. First, the bedrock data have been interpolated below the Quaternary cover; then a grid of geological sections has been drawn. This stage was coupled with a field control of the data, in order to detail the original data in complex areas and to collect mesostructural data to characterize the main surfaces. A 20x20m DTM (Regione Lombardia) was used for the topographic reconstruction. The 3D reconstruction, based on the surface data, geological sections and mesoscopic observation in selected sites, was produced with gOcad that can integrate this different information. The geometric model helps to define the cinematic evolution of the thrust system which is related to different tectonic stages.