Tectonic terraces in the Friulian plain (NE Italy)

The front of the Eastern Southalpine Chain (ESC) in Friuli is one of the most seismically active area of the Alps (Galadini et al., 2005); it is located at the margin between the Alpine foothills and the Friulian piedmont plain. The plain has formed by major river systems (Cellina, Meduna, Tagliamento and Torre rivers) mainly during the Quaternary cold phases. Most of the present surface of the plain can be ascribed to the Last Glacial Maximum (27 to 18 ky BP). Incisions into the plain related to the decay of the LGM and the Holocene interglacial are deep and located along the rivers and their tributaries. For this reason, most of the present terraces are located along the major rivers of the plain and follow their flow direction. But in the piedmont plain several terraces show a direction perpendicular to the rivers. These are in agreement with the tip line of the major mainly blind thrusts characterizing the front of the ESC. From the West to the East they can be resumed as follow: - The Aviano-Malnisio terraces is related to the Polcenigo-Montereale and Aviano-Budoia thrusts, which develops for 20 km along the foothill of the Mount Cavallo. They can be separated in different terrace systems: a) the Castello d'Aviano terrace that rises 30 m above the LGM Cellina fan and is made of cemented gravel of the Cellina river characterized by a well developed soil with Bt horizon 5YR (MUNSELL); b) the Marsure terraces rising also about 30 m over the Cellina LGM fan and made of cemented gravels of local fans, which display progressive angular-unconformity stack and lay on limestone bedrock; c) the Malnisio terrace across which LGM gravels are displaced of about 9 meters. - The Maniago Libero terrace, related to the Maniago Thrust, is confined in the left side of the Cellina valley outlet. It rises from the present riverbed of 90 m and about 40 m over the top of the LGM Cellina fan. It is made of weathered cemented gravels with a top soil 2.5 deep (Avigliano et al., 2002). - The Meduno terraces, related to the Maniago Thrust, are perpendicular to the other river terraces. A first terrace, mad of cemented gravel of the Meduna river, has a 15 m high scarp over the LGM fan; a second terrace, characterized by LGM gravels, has a 9 m high scarp and has developed from the LGM time to the present (Monegato and Poli, 2015). - The Valeriano and Rio Ponte terraces are related to the Arba-Ragogna thrust system (Poli et al., 2009) which deformed the LGM surface of the Tagliamento outwash plain with a 4 m high scarp. All these terrace systems, coupled with stratigraphic and structural analysis on the Quaternary successions, show the ongoing deformation along the front of the ESC with slip-rate estimations varying from 0.2 to 0.6 mm/yr. Recent investigations through paleoseismological trenches enlightened how some of these terraces have been interested by tectonic deformation also in correspondence of historical earthquakes.