Relazioni fra movimenti di versante e fenomeni tettonici nel bacino del Torrente Carpina (Umbria settentrionale)

Relationships between mass-movements and tectonic setting in the Carpina basin (Northern Umbria)

Abstract

A research project aimed at establishing the relationships between the geological and structural setting and the distribution of slope movements has been carried out in the Carpina basin, located in the northern sector of the Umbria region (Central Italy). The area is typical of the terrigenous Apennines of Central Italy, and was chosen for its geological and tectonic features as well as for the abundance and variety of slope movements. A 3500 m thick sequence of marl and shale, with interbedded sandstone and calcarenite, of Miocene age outcrops in the area. For the 67,22 km² basin we prepared a detailed structural-geological map at 1: 10,000 scale, a land-use map, and a detailed landslide inventory map, through the interpretation of aerial photographs of different vintages and extensive field surveys. The relationships between the fithologic and structural setting and the distribution of slope movements were studied. The presence of weak litological layers; the attitude of planar and continuous bedding planes dipping towards the free-face of slope: the presence of unfavourable hydrogeological conditions; and the occurrence of normal faults were identified as the most important factors controlling the distribution of slope movements in the area. According to movement typology, landslides were classified into three groups: surficial movements, on soil and marl; deep-seated recent and old, mostly translative, slides; and deepseated ancient slides in bedrock. According to the local tectonic setting four groups of landslides were identified: failures along slopes of the monocline valleys, where bedding dips either into or out of the slope; failures along slopes of the valleys cut perpendicularly to the monocline; failures along the thrust zones; and failures related to the presence of normal faults. Each group of mass-movements refers to a characteristic tectonic setting, inherited from the compressive deformations of Mio-Pliocene age, or from the extensional movements of Pliocene to Holocene age. Generally, slope movements were found to be the result of more than a single cause. The relationships between the different litological units outcropping in the area, the structural domains, each representing different tectonic setting, the land-use classes and the spatial distribution of landslides are also presented and discussed. For each theme classes more and less prone to failures are identified, both in absolute terms, computing the total landslide area, and in relative terms, computing the percentage of landslide area within each class. Lastly, a scheme of the tectonic and geomorphologic evolution of the area from the Pliocene to present is given, and the relationships between tectonic features and slope movements are briefly outlined.