QUATERNARY TECTONIC ACTIVITY IN THE WESTERN CARNIC PREALPS (EASTERN SOUTHERN ALPS, FRIULI).

In the framework of national research programs (CARG Project, DPC-INGV, S1 Project) detailed geological, structural and morphotectonic investigations integrated with the analysis of topographic maps (scale 1:10.000), high resolution-DEM (supplied by the Civil Protection of the Friuli Venezia Giulia Region) and ENI – AGIP seismic profiles, was carried out in the western sector of the Carnic Prealps (Friuli) in order to characterize as many as possible geometric (trend, size, depth), kinematic (slip vector, cumulative displacement, slip per event), dynamic (maximum expected magnitude) and chronological (slip-rate and mean recurrence interval) parameters of the major active faults located in this portion of the front of the eastern Southern Alps. The study area is located at the southern border of the western Carnic Prealps and includes the northern sector of the Friuli piedmont plain. This area belongs to the Pliocene-Quaternary front of the eastern Southalpine Chain (ESC), a south-verging polyphase fold and thrust belt, in evolution from the Middle Miocene to the Present (Castellarin et al., 1992). In the Carnic Prealps, the outer sector of the ESC is made of a series of arched SW-NE trending, SE-verging thin-skinned thrusts, stacking on the foreland and involving more external portions of the plain through the activation of new external splays. In the adjoining plain, thrusts are mostly blind but deformation affects the upper Pleistocene to Present fluvio-glacial and alluvial deposits. Presently in the Friuli area σ1 spreads between NNW-SSE in the western region, to NNE-SSW in the east. GPS records (Bechtold et al., 2009; Serpelloni et al., 2005) indicate a crustal shortening rate of about 2-3 mm/a. According to the DBMI Catalogue (Locati et al., 2011) the study area was hit by both historical and instrumental earthquakes: 1776 Tramonti, Io=8-9; 1812 Sequals, Imax: 7-8; 1873 Alpago Imax: 9-10; 1936 Cansiglio Imax: 9. Intensive losses were also recorded as a consequence of the destructive earthquakes of 1976 (Friuli) and 1928 (Carnia). Three tectonic units were identified and parametrized in the western pre-alpine Carnic area: the Arba-Ragogna, the M. Jouf-Maniago and the Polcenigo-Montereale respectively. a) The Mt. Jouf - Maniago is a NE-SW striking, SE-verging thrust-system running at the base of the Carnic prealps east the Cellina River. Both thrusts show evidence of Quaternary activity. Near Maniago locality the terraced middle and upper Pleistocene units (respectively Maniago conglomerates and Maniago gravels) are back-tilted and uplifted on the present piedmont plain (Zanferrari et al., 2008) and lacustrine deposits (9090 ± 90 years 14C BP), cropping out along the Colvera creek, a few kilometres north of Maniago, are gently folded and fractured (Zanferrari et al., 2008). Recently, on the basis of stratigraphic and morphotectonic analyses carried out on Pleistocene terraces at the outlet of the Meduna valley, a post-LGM-Holocene slip rate of about 0.5 mm/a was detected for the Maniago thrust (Monegato and Poli, 2015). b) South of the Maniago - M. Jouf tectonic unit, the Miocene succession is thrust and folded by the Arba-Ragogna thrust-system that shows evidence of Quaternary activity, as testifies the angular unconformity between the lower Messinian Montello Conglomerate and the lower Quaternary San Pietro di Ragogna Conglomerate (Zanferrari et al., 2008; Poli et al., 2009). The recent tectonic activity of the Arba-Ragogna thrust system is also testified by drainage anomalies and gentle scarps connecting uplifted paleolandscapes of Quaternary age (Galadini et al., 2005). A vertical slip-rate of about 0.19 mm/y has been calculated for the Arba–Ragogna thrust during the last 21 kys. (Poli et al., 2009). The analysis of the ENI-AGIP industrial seismic lines confirms evidence of Quaternary activity for both the structures. c) The Polcenigo-Montereale thrust-system gives rise to the major morphological element of the western area and it is made of a NW-SE striking imbricate thrusts running at the base of the Cansiglio-Cavallo carbonate plateau from Montereale Valcellina to Caneva. Toward the east it is segmented by the M. Jouf-Maniago thrust system, in the west it is separated from the Cansiglio thrust by means the Col Longone lateral ramp. The Polcenigo-Montereale thrust overlaps the Upper Jurassic - Upper Cretaceous Limestones of the Friulian Carbonatic Platform (FCP) not only on the Miocene Southalpine molassa but also on the Quaternary successions. Near Aviano the Holocene alluvial fan of Ossena river is gentle folded and its post-Roman age gravels are about 15° back-tilted. Near Polcenigo, the Budoia-Aviano splay deforms the LGM alluvial fan of the Artugna river giving rise to considerable drainage anomalies. Near Montereale along the Malnisio thrust the deposits related to the LGM Cellina fan (Avigliano et al., 2002) are displaced of about 10 m, suggesting also for this sector a recent slip rate of about 0.5 mm/y. West of Polcenigo, the Col Longone lateral ramp acts as left lateral strike-slip fault and splits the Polcenigo-Montereale thrust from the Cansiglio one. The W-E trending, S-verging low angle Cansiglio thrust extends for about 6 km around the base of the Cansiglio massif and overlaps the Upper Jurassic-Upper Cretaceous limestones of the FCP on the Miocene units. Late Quaternary activity is demonstrated by the displacement of a post-LGM stratified slope deposits along the fault surficial expression (Galadini et al., 2005). Westward it seals the Quaternary activity of Montello thrust (Galadini et al., 2005). On the basis of the processing of the macroseimic data-point distribution the Cansiglio thrust is generally considered as the source of the 1936 earthquake (Galadini et al., 2005; Sirovich et al., 2004).