The great landslide at Portella Colla (Madonie, Sicily)

The South Western area of the Madonie Mountains is affected by large landslides; the major one developed starting from Portella Colla down to the Imera Settentrionale river, for a maximum length of about 6.2 km in a NE-SW direction. The study of the landslide, with the aim of reconstructing the geological, geomorphological and evolutionary aspects, has been carried out using integrated methodologies, including some geophysical investigations (in particular geoelectrical prospecting). The landslide is complex and characterized by superficial and deep gravitational deformations. The movement began in the Upper Pliocene and it is still active. The origin and evolution of the landslide are linked to the geological structural setting of the area, as well as to the neotectonic activity and Quaternary climatic variations. The tectonic compressive phases of the Middle Miocene have determined the overlapping of mesozoic dolomitic-calcareous bodies on prevalently clayey rocks of Tertiary age. In the Plio-Pleistocene, the original structural building was fragmentaried during tectonic phases producing calcareous blocks. The geological setting and the instability caused by the increased relief energy, consequent to the last tectonic phase, constitute the main preparative causes for the beginning of the movement. In the Pleistocene, periods cooler than the actuals favoured a significant activity of the body of the landslide, due to the soil moisture. Furthermore, due to cryoclastic processes, an intensive degradation of the fractured limestones outcropping along the slopes has produced a large talus zone which partially covers the body of landslide. The main recent activity of the landslide is linked to exceptional meteorological events, as they occurred in 1931, causing the movement of the front of the landslide. This moved forward about 30 m, reaching the Imera Settentrionale river and diverting the bed of the Rio Secco river. At the same time some flow-rate variations and shifts of water springs on the body of the landslide have been observed. Another large movement occurred in 1959 in an area called C.da S. Venera, along the right side of the landslide. The present activity consists of modest movements, which generally occur with annual frequency; however, the permanent instability of the landslide confers to the whole area dangerous conditions, so that special constraints in the use of territory - as suggested by possible evolutionary scenarios - should be imposed