LARGE LANDSLIDES AND NEOTECTONICS: A CASE STUDY IN WESTERN SICILY

The study area is located in the western sector of the Sicanian Mounts where the superimposition, consequence of the Miocene and Lower Pliocene tectonic transpressive phases, of different geologic bodies, having vergence to the southern quadrants, can be observed. In this area, the neo-autochthonous deposits are given by marly-clayey sediments of middle-upper Pliocene, pertaining to the “Marnoso-arenacea” Formation of the Belice Valley, and by Pleistocene calcarenites. The recent extensional tectonic crisis (neotectonics) have further dissected and dismembered the original structural edifice, involving also the Pleistocene calcarenites, which show tectonic displacements mainly NE – SW oriented (Mascle, 1979; Catalano & D’Argenio, 1978; Calandra et alii, 1982; Di Stefano & Vitale, 1992; Monteleone et alii, 2006). The superimposition of the calcarenites, reaching a thickness of about 70 m, over the ductile portion of the “Marnoso-arenacea” Formation, determined favorable conditions for a geomorphological evolution of the western sector of the S.Margherita Belice flat area, that is mainly given by deep gravitational movements (DGSD); the triggering of the latter is to be correlated with the seismic activity that has, in historical and recent times, interested the Belice Valley. Therefore, the morphological irregularities of C.da Caliata have to be related to these deep gravitational movements; similar processes interest the area of C.da Saiarotto, where the combination with the presence of a large aquifer, which gives springs with total flow rates greater than 20 l/s, produced more intense movements in the middle-low portion of the landslide body, that reach the base of the slope, covering the bed of the River Belice and deviating the stream towards NW. The Deep-seated Gravitational Slope Deformations affecting this area have distinctive geomorphological features: in particular, disarticulated blocks, steps which are frequently reverse sloping, and areas with double ridges can be observed. The crowns of tilted blocks are often characterized by the presence of wide trenches, while, at the base of the collapsed masses, the accumulation zones frequently miss. Because of the partial rotation, the calcarenitic blocks have an upstream vergence and, sometimes, are covered by talus deposits with various thicknesses. The main scarp of the C.da Caliata landslide is dissected by the axis of the Vallone S. Nicola river that cuts the landslide body, deeply carving the Pleistocene biocalcarenites, and reaching, in the middle-low portion of the slope, the bedrock that is here given by clays and marly clays middle-upper Pliocene in age. The discovery of an archaeological site in the area affected by the DGSD of C.da Caliata, where two periods of habitation can be distinguished, one Byzantine (VII - VIII century) and the other one Islamic (XI century), leads to the assumption that the state of activity of the landslide body may have been quiescent since before the origin of the ancient settlements; otherwise it would be hard for the buildings to last for a so long period of time. The cause of abandonment of the site is probably to connect to the historical events that have occurred in this area (Agnesi & Monteleone, 1990). Field observations lead to consider these large mass movements, on the whole, not active; the few detected signs of activity are in fact to be linked to some rock falls in the head area and to few slide-flow type landslides affecting the medium-low portion of the large mass movements.